Fixes a problem where YouTube thumbnails may be missing and throw an error.
Adds additional ingest tests.
Ingest code halts early if there are problems.
Merge pull request #38 from webnard/bug/ingest-tests-squash

More ingest tests
Set the ma:duration attribute on the video asset model to have a default value of 0, to prevent an error when it tries to cast NoneType as int.
Added ma:duration to the disallowed attributes for the video asset model, so that it cannot be updated via the API (only the ingest functions will be able to set it).
Test to make sure the user can't set ma:duration to None
Merge branch 'master' of github.com:jlmcdonald/byu-hummedia-api
Test ensuring that ma:duration is set after ingesting a video.
Failing test for editing annotations as another user with write access.