Support validation messages for any xblock (on container page).

@andy-armstrong and @dan-f This is ready for review. The associated xblock PR has already been merged.

Known issues:
1) Updating xblock introduced 4 unit test failures due to the fact that the version of xblock in edx-platform was not the most recent. See #5825.
2) Still investigating bok choy test failure (the test passed for me locally).

I will create a sandbox and update when it is ready.
Note to reviewers: A test failure introduced by a recent refactoring in xblock repo.