Warn if plugin loader paths don't exist

This is a common misconfiguration for AUProximity users. Warn more explicitly

src/Impostor.Server/Plugins/PluginLoader.cs

@@ -24,6 +24,8 @@ public static IHostBuilder UsePluginLoader(this IHostBuilder builder, PluginConf
             // Add the plugins and libraries.
             var pluginPaths = new List<string>(config.Paths);
             var libraryPaths = new List<string>(config.LibraryPaths);
+            CheckPaths(pluginPaths);
+            CheckPaths(libraryPaths);
 
             var rootFolder = Directory.GetCurrentDirectory();
 
@@ -120,6 +122,18 @@ public static IHostBuilder UsePluginLoader(this IHostBuilder builder, PluginConf
             return builder;
         }
 
+        private static void CheckPaths(IEnumerable<string> paths)
+        {
+            foreach (var path in paths)
+            {
+                if (!Directory.Exists(path))
+                {
+                    Logger.Warning("Directory \"{path}\" was specified in the PluginLoader configuration, but this directory doesn't exist!", path);
+                }
+            }
+
+        }
+
         private static void RegisterAssemblies(
             IEnumerable<string> paths,
             Matcher matcher,