Users: Improved user response for failed invite sending

Added specific handling to show relevant error message when user
creation fails due to invite sending errors, while also returning user
to the form with previous input.
Includes test to cover.

For #5195

app/Access/UserInviteException.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace BookStack\Access;
+
+use Exception;
+
+class UserInviteException extends Exception
+{
+    //
+}