DM-35469: Make the storage class redefinition error message clearer

[timj]

Now uses repr() so that the differences are reported (str reported
just the name, which was already known to match)

Checklist

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

[codecov]

Codecov Report

Merging #706 (3b87561) into main (aeba0b4) will increase coverage by 0.00%.
The diff coverage is 88.88%.

@@           Coverage Diff           @@
##             main     #706   +/-   ##
=======================================
  Coverage   84.54%   84.54%           
=======================================
  Files         243      243           
  Lines       31814    31825   +11     
  Branches     5963     5966    +3     
=======================================
+ Hits        26896    26906   +10     
  Misses       3747     3747           
- Partials     1171     1172    +1     

Impacted Files	Coverage Δ
python/lsst/daf/butler/core/config.py	92.11% <50.00%> (-0.18%)	⬇️
python/lsst/daf/butler/core/storageClass.py	93.57% <90.90%> (+0.09%)	⬆️
tests/test_storageClass.py	98.91% <100.00%> (+0.02%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update aeba0b4...3b87561. Read the comment docs.

[timj]

@TallJimbo sorry, can you please take a second look at this. I cracked and decided to add the file name (where available) to the error message so we know which file is causing the problem (although I'm not yet retaining a list of all previous config files that have been read to provide further information).