Add support for error types

Add the possibility to add a type to an error. We might create an class for a validation error with a `__toString()` method. The class would have a `setType()` and `getType()` method.

The `ValidationResult::getErrors()` method should take a `$type` argument to only get errors of a specific type.

``` php
$validation = new ValidationResult();
$validation->addError("Failed parsing hello %s", "world")->setType('syntax');
```

Example use case:

``` php
$validation = $foo->validation();

if ($validation->failed()) {
if ($validation->getErrors('unique')) {
return $this->conflict($validation->getErrors('unique'));
} else {
return $this->badRequest($validation->getErrors());
}
}
```

Start by inspecting the ValidationResult class, especially addError() and getErrors(), and look for existing validation-result tests. Determine how errors are represented and how a typed error should be stringified. Done means errors can carry a type and getErrors($type) filters by it without breaking untyped retrieval.

Written by the indexing model from the issue text.