improve validation to provide file upload error messages

[dbu]

in #82 the checks during transformation are improved, but we should see if we can validate UploadFile earlier to provide decent messages to the user.

[dbu]

@webmozart can you maybe help us? we throw an exception in a transformer and would like to show that exception as the validation error message as we know that the exception message would be useful to the end user. https://github.com/symfony-cmf/MediaBundle/blob/master/Form/DataTransformer/ModelToFileTransformer.php#L50

it seems we only are able to get the generic error message however.

[rmsint]

Did have a look and I see a model normalization error causes the form to not be synchronized. The validator checks if a form is synchronized. See https://github.com/symfony/Form/blob/master/Form.php#L636 and https://github.com/symfony/Form/blob/master/Extension/Validator/Constraints/FormValidator.php#L98.

Maybe the config options invalid_message and invalid_message_parameters can be used to pass a specific upload error to the form? How could a model transformer do this?