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.

Tested running manage.py runserver without any dependency messages,

with dependency warnings only, with dependency errors only, and with

a mixture.
Help output was always printed when dependency-related messages were

logged, but only exited if there were errors.