DM-34215: Support dropping into debugger on exception

[PaulPrice]

When we get an exception, sometimes it's not enough to just
see the backtrace, but we also want to get a debugger prompt
at the point of the exception.

Replaced the --do-raise option (apparently unused, and often
associated with getting a debugger with Gen2) with --pdb
(well-known and appreciated option from pytest), which causes
MPGraphExecutor._executeQuantaInProcess to drop into pdb.

Checklist

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

[codecov]

Codecov Report

Merging #177 (7ec6ce7) into main (8174bdb) will decrease coverage by 0.21%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##             main     #177      +/-   ##
==========================================
- Coverage   81.37%   81.16%   -0.22%     
==========================================
  Files          41       41              
  Lines        3297     3308      +11     
  Branches      550      552       +2     
==========================================
+ Hits         2683     2685       +2     
- Misses        455      463       +8     
- Partials      159      160       +1     

Impacted Files	Coverage Δ
python/lsst/ctrl/mpexec/cli/opt/optionGroups.py	100.00% <ø> (ø)
python/lsst/ctrl/mpexec/cli/script/run.py	55.55% <ø> (ø)
python/lsst/ctrl/mpexec/cmdLineFwk.py	60.63% <ø> (ø)
python/lsst/ctrl/mpexec/mpGraphExecutor.py	86.75% <18.18%> (-2.74%)	⬇️
python/lsst/ctrl/mpexec/cli/opt/options.py	82.69% <100.00%> (ø)

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[andy-slac]

Looks good, couple of minor comments.