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 
Do we need to specifically not call the old error handler?  Can't we get it, then override it to emit, then call the old one?

```php
$oldHandler = set_error_handler(function ($errno, $errstr, $errfile, $errline) {});
set_error_handler((function ($errno, $errstr, $errfile, $errline) use ($oldHandler) {
    $this->emit('error', [$errno, $errstr, $errfile, $errline]);
    if ($oldHandler) {
        $oldHandler($errno, $errstr, $errfile, $errline);
    }
});
```

Kinda sucks there is no get_error_handler(), so first have to set dummy handler.

Thoughts?