Better message for required fileds in form validation

[codefisher]

When a required field fails validation is says "This field is required". I think it should say something more like "Subject is required", assuming the label of the field is "Subject". It in my opinion is more user friendly.

I know this can be done by changing the error_messages argument/variable. But that means doing it for possibly hundreds of fields.

I will attach a patch, hope I get it right, it will be the first time ever I have posted a patch.

[Tim Graham]

Thanks for the suggestion, but there are backwards compatibility concerns. Everyone using error_messages['required'] key would need to start passing 'label' in params. I don't think the gains outweigh the drawbacks.