Engine-Twitter: set an error message when the user is not found

Set an error message when the user is not found, otherwise the
error message shown to the user as "Reason" would be the useless
"Exception of type 'Twitterizer.TwitterizerException' was thrown.",
when launching a command such as /timeline non-existing-username.

src/Engine-Twitter/Protocols/Twitter/TwitterProtocolManager.cs

@@ -1818,6 +1818,9 @@ private void CheckResponse<T>(TwitterResponse<T> response) where T : ITwitterObj
             if (response.Result == RequestResult.Unknown &&
                 response.ErrorMessage == null) {
                 response.ErrorMessage = _("Twitter didn't send a valid response, they're probably overloaded");
+            } else if (response.Result == RequestResult.FileNotFound &&
+                response.ErrorMessage == null) {
+                response.ErrorMessage = _ ("Twitter user not found");
             }
             throw new TwitterizerException(response.ErrorMessage);
         }