Exposes Pretty property of CommandAppException

If a user is propagating the exceptions we aren't going to automatically display them. They might still want to get this exception message and use it, so we'll make it publicly available.

src/Spectre.Console.Testing/Cli/CommandAppTester.cs

@@ -53,6 +53,15 @@ namespace Spectre.Console.Testing
             }
             catch (T ex)
             {
+                if (ex is CommandAppException commandAppException && commandAppException.Pretty != null)
+                {
+                    console.Write(commandAppException.Pretty);
+                }
+                else
+                {
+                    console.WriteLine(ex.Message);
+                }
+
                 return new CommandAppFailure(ex, console.Output);
             }
             catch (Exception ex)