[ParamFetcher] Customize ConstraintViolation errors

[pyrech]

The feature introduced with #700 is really awesome. However, currently, the violation message returned by the validator is not really nice to output to an api client:

"the value given:\n the violation message\n"

This output is caused by: ConstraintViolation::__toString() and ConstraintViolationList::__toString(), called by throw new BadRequestHttpException((string) $errors); in ParamFetcher::cleanParamWithRequirements()

It could be better to display at least the name of the param which failed, if possible in first position, and without the "\n" char between value and message. An even better way would be to allow the customization of the message, what to you think about it?

[lsmith77]

can you work on a PR?

[pyrech]

I will give it a look tonight ;)

[pyrech]

Closing in favor of #734