Fix dependency warnings preventing manage.py from running.
Review Request #9562 — Created Feb. 1, 2018 and submitted
A recent change added improved dependency warning/error output and tracking, but due to a design error from a later iteration of this change, we were treating warnings as errors, exiting the process when optional dependencies weren't found. This change updates this code to track warnings and errors separately, printing help information whenever there are any dependency-related messages printed but only exiting if there were errors.
manage.py runserverwithout any dependency messages,
with dependency warnings only, with dependency errors only, and with
Help output was always printed when dependency-related messages were
logged, but only exited if there were errors.