Make environment error message be a non-empty string or None

[callumforrester]

The code was using None or "" in various places to denote the lack of an error message. None is a better semantic fit, so this PR forces the user to pass either None or a string with at least 1 character.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.21%. Comparing base (3461bcc) to head (d8c1829).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #571   +/-   ##
=======================================
  Coverage   90.21%   90.21%           
=======================================
  Files          41       41           
  Lines        1829     1829           
=======================================
  Hits         1650     1650           
  Misses        179      179           

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