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).

Sandbox-- http://studio.cahrens.m.sandbox.edx.org/course/validation/validation/validation (created as "verified").

@explorerleslie Please take a look at my sandbox.
I actually now think this whole condition can just be `if (!summary.text)`. This handles: `summary` having no field `text` as well as `summery.text` existing but being falsy (null, undefined, empty string, etc.). The `summary.type` check can follow the same pattern.