ENH: Add function to say why a call was not in the cache

[larsoner]

Consider this proposal-as-PR, feel free to close if out of scope or deemed not sufficiently useful. I was just using this for local debugging so thought it's possible others might need it, too.

I was getting some cache misses and didn't understand why. This adds a function to give a reason string why. I assumed it was a hash mismatch, but it's nice to know that joblib saw it that way so I could confidently debug the issue at my end.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.30%. Comparing base (4273f39) to head (f3b529a).

Files with missing lines	Patch %	Lines
joblib/memory.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1763      +/-   ##
==========================================
+ Coverage   94.20%   94.30%   +0.10%     
==========================================
  Files          46       46              
  Lines        7921     7924       +3     
==========================================
+ Hits         7462     7473      +11     
+ Misses        459      451       -8     

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