Support for webform custom messages

There's an issue with patches on the webform module to add support for custom validation messages. At the moment this isn't merged, and I'm unsure how likely it is that it will be, but I have a need to use this functionality on a project, and am therefore attaching a patch which will allow clientside_validation_webform to display the custom error message set for a field if available. I don't think this should be merged into the codebase unless that issue in webform is merged first, but others may find it useful if they are also rolling the patch from #889198

Cheers,
Sam