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 
What's the point of this?  The SUT code only executes in the context of the three lines above.  Also, see above comment.  If we keep it the way it is, this can be one line:

```php
restore_error_handler();
```