I've noticed that sometimes when our CI server is under high load, we were getting an intermittent test failure in ExtensionManagerTests.test_install_media_concurrent_threads. This lead me to discover that in Django 3.2, `django.core.files.locks.lock()` changed its interface to return `False` instead of raise `IOError`. This meant that our use of this has not actually been using the lock at all. This change updates the extension manager to check the return value and raise an IOError to restore the behavior described in the docstring. Testing Done: - Ran unit tests. - Artificially created a high load situation and verified that prior to the fix we would hit contention, and after we properly respect the lock. - Grepped through our codebases to ensure there weren't any other similar bugs.