Input error messsages base.
Improve error feedback in authoring forms

Details:
1. Add client-side validation error feedback for authoring forms (timeline, exhibit, contentitem);
2. Add server-side validation error feedback for MediaSource input in authoring forms (exhibit, contentitem);
3. Disabled obsolete authoring tests (alert window on error)
Bugfix #682
Merge pull request #1090 from alex-zen/error-messages

Error messages