Fix for issue 5090

https://github.com/diaspora/diaspora/issues/5090

Changing error message wording and providing more information to user when status message is too long. I can add a test if you'd like, but none existed before. All tests pass. I'm not a huge fan of the controller change (@status_message.errors.messages[:text].join('. ')) as it looks clunky and the model change (adding a proc to the validates message) looks odd, but that seems to be the way to add the current message length.

I think this also will fix https://github.com/diaspora/diaspora/pull/5099 as it appears to be similar if not identical