Improve validation error messages

[kevinrobell-st]

Currently the error message that is shown when an email is invalid is unspecific. It'd be a great improvement for it to explain why the email is invalid: disposable domain, disposable mx server, invalid syntax, deny list, etc. This would help with debugging.

[phoet]

I agree. It would make a lot of sense to make it inspectable.

[kwent]

Also i had like to override the default one when it's done through i18n. etc.

[phoet]

I'm currently not active working on the project. If anyone wants to go for it, I think the EmailValidator class should be a good starting point.