Add warning when jobflow.yaml is blank or badly formatted

[mcgalcode]

Summary

Addresses #411, and adds moto as a testing dependency.

Additional dependencies introduced (if any)

• moto is added as a test dependency, but this is because it is already used in a test in tests/core/test_store.py

Checklist

Work-in-progress pull requests are encouraged, but please put [WIP] in the pull request
title.

Before a pull request can be merged, the following items must be checked:

• Code is in the standard Python style.
  The easiest way to handle this is to run the following in the correct sequence on
  your local machine. Start with running black on your new code. This will
  automatically reformat your code to PEP8 conventions and removes most issues. Then run
  pycodestyle, followed by flake8.
• Tests have been added for any new functionality or bug fixes.
• All linting and tests pass.

Note that the CI system will run all the above checks. But it will be much more
efficient if you already fix most errors prior to submitting the PR. It is highly
recommended that you use the pre-commit hook provided in the repository. Simply
cp pre-commit .git/hooks and a check will be run prior to allowing commits.

[codecov]

Codecov Report

Merging #415 (ee1e109) into main (1647e53) will not change coverage.
Report is 4 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #415   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           19        19           
  Lines         1502      1508    +6     
  Branches       376       412   +36     
=========================================
+ Hits          1502      1508    +6     

Files Changed	Coverage Δ
src/jobflow/settings.py	100.00% <100.00%> (ø)

[utf]

Thanks very much. Only thing is why the test isn't actually catching the value error.

[utf]

Thanks very much!