Assert::error() prints all unexpected errors.

Useful when using the Assert::error() and callback generates for example E_NOTICE before expected error:
```php
Assert::error(function() {
    $a++;
    trigger_error('Bad file content', E_USER_WARNING);
}, E_USER_WARNING, 'Bad file content');
```
Current behaviour is:
```
-- FAILED: Test | test.phpt
   Unexpected error Bad file content in file.php:123
```