Quiet down exceptions at startup, and during tests

There's no need to display a traceback for every failed content load,
the comment before the log line even says so.

The exceptions shown before tests are run are because of the eager
initialization of the modulestores.  They don't need to be initialized
then, that just speeds the responsiveness of servers.  Putting off the
initialization means they get inited as needed, and the log lines get
swallowed by the test runner.

@cdodge @wedaly Thoughts?