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