Assert::error() prints all unexpected errors.

Useful when using Assert::error() and callback throw e.g. E_NOTICE before expected error:
```php
Assert::error(function() use ($o) {
    $o->setFile($file); # Undeclared $file
}, E_WARNING, 'Bad file content');
```
Current behaviour is:
```
-- FAILED: Test | test.phpt
   Unexpected error Bad file content in file.php:123
```