Improve connection tests

The ajax-based connection tests in the mailbox configuration can be a bit flaky... I should figured out how to make them more user-friendly and robust. A good start might be to call drupal_get_messages() to clear existing messages whenever a connection test is performed to prevent imap errors from accumulating in the message queue, and then if an error is returned on the ajax test advise users to save the form and details will be displayed on the next page.