Implement wrapException to generalize parseConfigFileSimple

The need for parseConfigStringSimple recently arised in dub,
and rather than implementing yet another function,
we can generalize what parseConfigFileSimple is doing,
allowing us to reuse the logic without introducing
a new function.

source/configy/Read.d

@@ -287,18 +287,48 @@ public struct CLIArgs
 
 public Nullable!T parseConfigFileSimple (T) (string path, StrictMode strict = StrictMode.Error)
 {
-    return parseConfigFileSimple!(T)(CLIArgs(path), strict);
+    return wrapException(parseConfigFile!(T)(CLIArgs(path), strict));
 }
 
-
 /// Ditto
-public Nullable!T parseConfigFileSimple (T) (in CLIArgs args, StrictMode strict = StrictMode.Error)
+Nullable!T parseConfigFileSimple (T) (in CLIArgs args, StrictMode strict = StrictMode.Error)
 {
-    try
+    return wrapException(parseConfigFile!(args, strict));
+}
+
+/*******************************************************************************
+
+    Wrap and print exceptions to stderr
+
+    This allows to call either `parseConfigFile` or `parseConfigString`
+    and pretty-print the exception:
+    ```
+    int main ()
     {
-        Node root = Loader.fromFile(args.config_path).load();
-        return nullable(parseConfig!T(args, root, strict));
+        auto configN = wrapException(
+            parseConfigString!Config("config.yaml", "/dev/null")
+        );
+        if (configN.isNull()) return 1; // Error path
+        auto config = configN.get();
+        // Rest of the program ...
     }
+    ```
+
+    Params:
+        parseCall = A call to one of the `parse*` functions, such as
+                    `parseConfigString` or `parseConfigFile`, or anything
+                    that would call them.
+
+    Returns:
+        An initialized `T` instance if reading/parsing was successful;
+        a `null` instance otherwise.
+
+*******************************************************************************/
+
+public Nullable!T wrapException (T) (lazy T parseCall)
+{
+    try
+        return nullable(parseCall);
     catch (ConfigException exc)
     {
         exc.printException();