Checklist

• I have checked the issues list for similar or identical enhancement to an existing feature.
• I have checked the commit log to find out if a the same enhancement was already implemented in master.

Brief Summary

Following the breakage caused by #4990 which was reported by #5383 it seems that we need more coverage to our CLI.

Design

I suggest that we take a BDD approach to these tests.
BDD tests describe the behaviour a user expects when he does something.

For example in this case:

Scenario: A user tries to generate a report and specifies a Celery app that does not exist
  Given a user tries to load a Celery app that does not exist
  When they run `celery -A doesnotexist report`
  Then the following error message is printed in red:
     """
     Error: 
     Unable to load celery application.
     The module doesnotexist was not found.
     """

or something along those lines.

Architectural Considerations

We'll need to install pytest-bdd as part of the test suite.

Proposed Behavior

None

Proposed UI/UX

Whatever pytest-bdd provides us with.

Diagrams

N/A

Alternatives

Write this tests without BDD.
They will be less clear to the user.