SimpleTest does not map PHP errors that occur during curl requests to exceptions.

As I noted in #329225: PHP Warnings when adding configurable actions., we're missing any PHP errors that occur during tests. These should ideally be mapped to exceptions. One way to do this would be to have a fake module that is always enabled during all simpletests that overrides drupal_error_handler; another way would be to hijack drupal_error_handler() itself; a third way would be to read any PHP errors that occur that are logged to the watchdog table. We could definitely discover a lot of errors that are occurring in this manner, which will be beneficial overall. I would not be surprised if mapping such errors to exceptions revealed several bugs in Drupal.