fix: Incorrect credentials are masked by traceback

[sadsfae]

• find_session_uri masks a proper error message when someone passes incorrect credentials (either by vars or user/pass).
• response body seems to be a JSON object and not a redfish root object and we are not generating a useful error, instead the user gets a traceback that masks the real cause.

before (see #517)

after

-=>>PYTHONPATH="./src" python3 src/badfish/main.py -H mgmt-d23-h31-000-r650.example.com -u root -p awrongpassword --power-state
- WARNING  - Passing secrets via command line arguments can be unsafe. Consider using environment variables (BADFISH_USERNAME, BADFISH_PASSWORD).
- ERROR    - Failed to authenticate. Verify your credentials for mgmt-d23-h31-000-r650.example.com

• Also remove asyncio import from tests/test_main_coverage.py as it wasn't being utilized.

fixes: #517

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.09%. Comparing base (3de528b) to head (54fa694).
⚠️ Report is 3 commits behind head on development.

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #518      +/-   ##
===============================================
+ Coverage        92.07%   92.09%   +0.01%     
===============================================
  Files                8        8              
  Lines             2511     2516       +5     
===============================================
+ Hits              2312     2317       +5     
  Misses             199      199              

Flag	Coverage Δ
unittests	92.09% <100.00%> (+0.01%)	⬆️

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