Wrap warnings as well

[bfabio]

Right now the ValidationException carries the errors like this, but not the warnings (eg. deprecated fields):

try {
    $parser = new Parser();
    $publicCode = $parser->parseFile('/path/to/publiccode.yml');
} catch (ValidationException $e) {
    $e->getErrors()
}

We should have a way to access the warnings as well, maybe via the PublicCode object (publiccode.getWarnings(): ?array) ?