Update error output formatting

Sources/Backend/FileManagement/FileVerifier.swift

@@ -68,7 +68,7 @@ struct FileVerifier {
         case isNotDirectory(String)
         case invalidPathExtension(String, FileType?)
 
-        var message: FormattedText {
+        var errorMessage: FormattedText {
             switch self {
             case .alreadyExists(let path):
                 return "'\(path, color: .yellow)' already exists"
@@ -89,6 +89,15 @@ struct FileVerifier {
                 return start + " for unknown output type"
             }
         }
+
+        var fix: FormattedText? {
+            if case .invalidPathExtension(_, let outputType) = self {
+                if let type = outputType?.preferredFilenameExtension {
+                    return "Use path extension '\(type, color: .green, style: .bold)'"
+                }
+            }
+            return nil
+        }
     }
 
     /// The options that specify the verifications to perform.

Sources/Backend/ImageProcessing/Iconset.swift

@@ -81,7 +81,7 @@ extension Iconset {
         private enum IconValidationError: String, FormattedError {
             case invalidDimensions = "Image width and height must be equal."
 
-            var message: FormattedText {
+            var errorMessage: FormattedText {
                 "\("Invalid icon", color: .red) — \(rawValue, style: .bold)"
             }
         }

Sources/Backend/ImageProcessing/ProcessingError.swift

@@ -14,7 +14,7 @@ enum ImageProcessingError: String, FormattedError {
     case pdfDocumentError = "Error with PDF document."
     case svgCreationError = "Error creating image data from SVG."
 
-    var message: FormattedText {
+    var errorMessage: FormattedText {
         "\("Could not process image", color: .red) — \(rawValue, style: .bold)"
     }
 }

Sources/Backend/Runners/MainRunner.swift

@@ -3,6 +3,8 @@
 //  createicns
 //
 
+import Darwin
+
 /// The main runner that encapsulates the behavior of the command, delegating
 /// parts of its execution to additional sub-runners.
 public struct MainRunner: Runner {
@@ -40,10 +42,29 @@ public struct MainRunner: Runner {
         }
     }
 
-    public func run() throws {
-        try validate()
-        for runner in runners {
-            try runner.run()
+    public func run() {
+        do {
+            try validate()
+            for runner in runners {
+                try runner.run()
+            }
+        } catch {
+            exit(with: error)
+        }
+    }
+
+    func exit(with error: Error) -> Never {
+        let box = FormattedErrorBox(error: error)
+        let errorText = FormattedText("error:", color: .red, style: .bold)
+            .appending(" ")
+            .appending(box.errorMessage)
+        print(errorText)
+        if let fix = box.fix {
+            let fixText = FormattedText("fix:", color: .green, style: .bold)
+                .appending(" ")
+                .appending(fix)
+            print(fixText)
         }
+        Darwin.exit(EXIT_FAILURE)
     }
 }

Sources/Backend/Utilities/Errors.swift

@@ -10,16 +10,48 @@ import Foundation
 /// An error type that is displayed in a formatted representation when printed
 /// to a command line interface.
 public protocol FormattedError: Error, TextOutputStreamable {
-    /// The formatted message to display.
+    /// The formatted error message to display.
     ///
-    /// If one of either standard output or standard error does not point to a
-    /// terminal, the message is displayed without formatting.
-    var message: FormattedText { get }
+    /// If one of either standard output or standard error does not point to 
+    /// a terminal, the message is displayed without formatting.
+    var errorMessage: FormattedText { get }
+
+    /// An optional formatted message to display that describes how the user
+    /// can remedy this error.
+    ///
+    /// If one of either standard output or standard error does not point to
+    /// a terminal, the message is displayed without formatting.
+    var fix: FormattedText? { get }
+}
+
+extension FormattedError {
+    public var fix: FormattedText? { nil }
 }
 
 extension FormattedError {
     public func write<Target: TextOutputStream>(to target: inout Target) {
-        message.write(to: &target)
+        errorMessage.write(to: &target)
+    }
+}
+
+// MARK: FormattedErrorBox
+
+/// A formatted error type that wraps another error.
+struct FormattedErrorBox: FormattedError {
+    let error: any Error
+
+    var errorMessage: FormattedText {
+        if let error = error as? FormattedError {
+            return error.errorMessage
+        }
+        return FormattedText(contentsOf: error.localizedDescription)
+    }
+
+    var fix: FormattedText? {
+        if let error = error as? FormattedError {
+            return error.fix
+        }
+        return nil
     }
 }
 
@@ -34,7 +66,7 @@ struct ContextualDataError: FormattedError {
     /// A string describing the context in which this error occurred.
     let context: String
 
-    var message: FormattedText {
+    var errorMessage: FormattedText {
         var message = FormattedText(context + ":", color: .yellow, style: .bold)
         if let string = String(data: data, encoding: .utf8) {
             message.append(" \("An error occurred with the following data:", color: .red) \(string)")

Sources/Frontend/CreateICNS.swift

@@ -19,8 +19,8 @@ struct CreateICNS: ParsableCommand {
 
     @OptionGroup var options: Options
 
-    func run() throws {
-        try MainRunner(
+    func run() {
+        MainRunner(
             input: options.input,
             output: options.output,
             type: options.type,