Added missing error messages for fetchmail form.

interface/web/mail/lib/lang/en_mail_get.lng

@@ -8,4 +8,8 @@ $wb["source_delete_txt"] = 'Delete emails after retrieval';
 $wb["destination_txt"] = 'Destination';
 $wb["active_txt"] = 'Active';
 $wb["limit_fetchmail_txt"] = 'The max. number of Fetchmail records for your account is reached.';
+$wb["source_server_error_isempty"] = 'Server is empty.';
+$wb["source_username_error_isempty"] = 'Username is empty.';
+$wb["source_password_error_isempty"] = 'Password is empty.';
+$wb["destination_error_isemail"] = 'No destination selcted.';
 ?>
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