DM-45722: CRITICAL logs on empty quantum graph

[kfindeisen]

This PR fixes a display bug introduced in #305, where pipetask's exit message referred to explanatory logs as CRITICAL instead of ERROR.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.00%. Comparing base (1162e18) to head (d69df87).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
python/lsst/ctrl/mpexec/cli/cmd/commands.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #306      +/-   ##
==========================================
- Coverage   89.02%   89.00%   -0.03%     
==========================================
  Files          50       50              
  Lines        4439     4439              
  Branches      733      733              
==========================================
- Hits         3952     3951       -1     
  Misses        347      347              
- Partials      140      141       +1     

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