Catch InvalidKeyError in course_id_from_url().

LMS-9663
Incomplete course ids should return a 404.
Merge pull request #4736 from edx/usman/lms11205-embargomiddleware-invalidkeyerror

Catch InvalidKeyError in course_id_from_url().
BLD-1125: Validation of content experiments after managing a group.
Added styling.
Added Jasmine tests.
REVERT THIS BEFORE MERGE.
Add message type.
Different icons depending on type of notification message (error or warning).
Modified Jasmine tests to reflect new functionality.
Have validation message display same string as the one on unit page.
Modified Jasmine tests again to reflect change in functionality.
Added incomplete bok-choy tests.