Improved input validation error messages for values from env variables

[dskrypa]

In terms of the publicly-facing API, nothing changed in this PR, except for the error messages presented to end users when input validation fails for values that were provided via environment variables. Now, when an invalid value is found in an environment variable, the name of the environment variable that contained the invalid value is included in the error message. This should make it easier to troubleshoot errors caused by forgotten environment variables and other similar cases.

In the internal API, potentially breaking changes were made to some internal methods related to ParamActions and their interaction with Parameters while parsing / validating values. Unused cruft was removed, a method was refactored to move away from having a param used for a single purpose to having a separate method for that purpose, and the new env_var param was added so it could be included in error messages.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.00%. Comparing base (7db5287) to head (71342d4).
Report is 24 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #78   +/-   ##
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  Coverage   98.00%   98.00%           
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  Files          49       49           
  Lines        6772     6780    +8     
  Branches     1064     1065    +1     
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+ Hits         6637     6645    +8     
  Misses        110      110           
  Partials       25       25           

Flag	Coverage Δ
unittests	98.00% <100.00%> (+<0.01%)	⬆️

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