hg: return 400 Bad Request for hg commands that not are commands Avoid throwing bare Exceptions which requires framework specific testing. Instead, return a reasonable http error code and make the test more framework independent. The "helpful" message will just be a description of the http exception and not sent to the client.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.