allow custom validation messages without pattern

[davemo]

• I wanted to be able to allow custom validation messages but not have to rely on client-side i18n and the use of the pattern attribute.
• I’ve got a failing test here because I’m not super familiar with how your testing setup is expected to work, I was hoping to be able to verify the arguments of the Validity constructor, but it's private. I'm including the test in the hopes it reveals enough intent that you can help me out a bit with how to get this test passing, the code changes as they are work for this feature :)

[chemerisuk]

@davemo the standard says that title is respected only if the pattern attribute exists. We should think twise when trying to tweak the standard. Could you explain your use case in more details?