Nicer error message when before insert/update trigger fails

[malaire]

This is probably easy to fix while fixing #74 but since this is a bit different, I created separate issue.

I have BEFORE INSERT and BEFORE UPDATE triggers on table which prevent some invalid entries with SIGNAL SQLSTATE '45000'. For example:

CREATE TRIGGER `PageRedirect_bi` BEFORE INSERT ON `PageRedirect`
FOR EACH ROW
  # UrlName must also be unique with Page.UrlName
  IF (EXISTS(SELECT * FROM Page WHERE UrlName = NEW.UrlName)) THEN
    SIGNAL SQLSTATE '45000';
  END IF

When trying to save new or updated record with invalid values, Xataface dies with uncaught exception instead of handling this gracefully.

PHP Fatal error: Uncaught exception 'Exception' with message 'Failed to update due to sql error: Unhandled user-defined exception condition' in
/xataface-2.1.3/Dataface/IO.php:1033
Stack trace:

0 /xataface-2.1.3/Dataface/IO.php(678): Dataface_IO->_update(Object(Dataface_Record), Array, 'PageRedirect', true)

1 /xataface-2.1.3/Dataface/QuickForm.php(1142): Dataface_IO->write(Object(Dataface_Record), Array, NULL, true, false)

2 [internal function]: Dataface_QuickForm->save(Array)

3 /xataface-2.1.3/lib/HTML/QuickForm.php(1632): call_user_func(Array, Array)

4 /xataface-2.1.3/actions/edit.php(130): HTML_QuickForm->process(Array)

5 /xataface-2.1.3/Dataface/Application.php(2426): dataface_actions_edit->handle(Array)

6 /xataface-2.1.3/Dataface/Application.php(2578): Dataface_Application->handleRequest(false)

7 /xataface-2.1.3/Dataface/Application.php(2447): Dataface_Application->_display(false, false)

...