Suppress php errors and add error event

This PR suppresses php errors via a custom error handler. Currently they are not reported, but a runner event is raised known as error:

```php
$oldHandler = set_error_handler(function($errno, $errstr, $errfile, $errline) {
    $this->emit('error', [$errno, $errstr, $errfile, $errline]);
});
```

Though no steps have been taken to report these now, any custom plugin or reporter could do something meaningful with this event.

Fixes #32 
I did not realize this was a thing. This is cool, lets remove the if and just call `restore_error_handler`. I think there is a point in doing this from a library perspective rather than a CLI perspective.

You are right, in the context of the cli executing a test suite, the previous error handler will count for little. However, if people build code **arround** peridot, put things back where we found them seems to be the responsible thing to do.