(#606) added ExceptionHandler to ICommandAppSettings

So exceptions can be handled in Spectre.Console.Cli.
Also, some documentation was added.

docs/input/cli/exceptions.md

@@ -0,0 +1,116 @@
+Title: Exceptions
+Order: 12
+Description: "Handling exceptions in *Spectre.Console.Cli*"
+---
+
+Exceptions happen. 
+
+`Spectre.Console.Cli` handles exceptions, writes a user friendly message to the console and sets the exitCode
+of the application to `-1`.
+While this might be enough for the needs of most applications, there are some options to customize this behavior.
+
+## Propagating exceptions
+
+The most basic way is to set `PropagateExceptions()` during configuration and handle everything.
+While this option grants the most freedom, it also requires the most work: Setting `PropagateExceptions`
+means that `Spectre.Console.Cli` effectively re-throws exceptions.
+This means that `app.Run()` should be wrapped in a `try`-`catch`-block which has to handle the exception
+(i.e. outputting something useful) and also provide the exitCode (or `return` value) for the application.
+
+```csharp
+using Spectre.Console.Cli;
+
+namespace MyApp
+{
+    public static class Program
+    {
+        public static int Main(string[] args)
+        {
+            var app = new CommandApp<FileSizeCommand>();
+
+            app.Configure(config =>
+            {
+                config.PropagateExceptions();
+            });
+
+            try
+            {
+                return app.Run(args);
+            }
+            catch (Exception ex)
+            {
+                AnsiConsole.WriteException(ex, ExceptionFormats.ShortenEverything);
+                return -99;
+            }
+        }
+    }
+}
+```
+
+## Using a custom ExceptionHandler
+
+Using the `SetErrorHandler()` during configuration it is possible to handle exceptions in a defined way.
+This method comes in two flavours: One that uses the default exitCode (or `return` value) of `-1` and one
+where the exitCode needs to be supplied.
+
+### Using `SetErrorHandler(Func<Exception, int> handler)`
+
+Using this method exceptions can be handled in a custom way. The return value of the handler is used as
+the exitCode for the application.
+
+```csharp
+using Spectre.Console.Cli;
+
+namespace MyApp
+{
+    public static class Program
+    {
+        public static int Main(string[] args)
+        {
+            var app = new CommandApp<FileSizeCommand>();
+
+            app.Configure(config =>
+            {
+                config.SetExceptionHandler(ex =>
+                {
+                    AnsiConsole.WriteException(ex, ExceptionFormats.ShortenEverything);
+                    return -99;
+                });
+            });
+
+            return app.Run(args);
+        }
+    }
+}
+```
+
+### Using `SetErrorHandler(Action<Exception> handler)`
+
+Using this method exceptions can be handled in a custom way, much the same as with the `SetErrorHandler(Func<Exception, int> handler)`.
+Using the `Action` as the handler however, it is not possible (or required) to supply a return value.
+The exitCode for the application will be `-1`.
+
+```csharp
+using Spectre.Console.Cli;
+
+namespace MyApp
+{
+    public static class Program
+    {
+        public static int Main(string[] args)
+        {
+            var app = new CommandApp<FileSizeCommand>();
+
+            app.Configure(config =>
+            {
+                config.SetExceptionHandler(ex =>
+                {
+                    AnsiConsole.WriteException(ex, ExceptionFormats.ShortenEverything);
+                });
+            });
+
+            return app.Run(args);
+        }
+    }
+}
+```