Increase task_class input validation and log messages

[backmari]

Description of the changes

Workflow Manager postprocessing tasks can optionally be defined by giving a "task class" in the database table report_task. This change improves the input validation and log messages for the task class entry.

Check all that apply:

• updated documentation
• Source added/refactored
• Added unit tests
• Added integration tests
• (If applicable) Verified that manual tests requiring the /SNS and /HFIR filesystems pass without fail

References:
Story 6374: [WebMon] Improve workflow manager "task_class" error

Manual test for the reviewer

(Instructions for testing here)

Check list for the reviewer

• best software practices
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.77%. Comparing base (b6b0d49) to head (6d6362b).

Additional details and impacted files

@@            Coverage Diff             @@
##             next     #191      +/-   ##
==========================================
+ Coverage   77.52%   77.77%   +0.24%     
==========================================
  Files          54       54              
  Lines        4704     4720      +16     
==========================================
+ Hits         3647     3671      +24     
+ Misses       1057     1049       -8     

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