Improve error handling configuration

Errors of type E_USER_ERROR, thrown by service_error(), are currently not returned if you change the error level of the amfphp module to E_ERROR. However, this does not allow the Flash frontend anymore to give the end user some reasonable clues about what went wrong.

For example, registering a user with user.save with a username that already exists, will just return NULL now. When you put the error level to E_ALL ^ E_NOTICE it returns something more useful:

array(5) {
  ["description"]=>
  string(197) "The name <em>testerdetest</em> is already taken.
<p>
	Het e-mailadres <em>kristof+testerdetest@statik.be</em> is al geregistreerd. <a href="/user/password">Ben je je wachtwoord vergeten?</a></p>
"
  ["details"]=>
  string(84) "/var/www/dev-multisite/sites/dev.uitmetvlieg.be/modules/contribute/amfphp/amfphp.inc"
  ["level"]=>
  string(18) "Unknown error type"
  ["line"]=>
  float(74)
  ["code"]=>
  string(20) "AMFPHP_RUNTIME_ERROR"
}

However, I do not want E_WARNING errors to be handled by amfphp as well, as some modules we are using cause E_WARNING errors (not good, I know, although it are the latest stable versions of contributed modules though) and the web service becomes unusable.

Now there is just a dropdown with two choices for error handling, but I suggest to put separate checkboxes in the settings interface for each type of PHP error, so users can just enable those specific errors they want to see.

I am willing to contribute a patch for this if there is interest.