keystore: improve error messages and include key file name

[gammazero]

Allows easier debugging and visibility into which keystore file is corrupted so that it can be removed.

See: ipfs/kubo#11102

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.04%. Comparing base (f76a2a7) to head (c9cb61d).

Files with missing lines	Patch %	Lines
keystore/keystore.go	75.00%	4 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main    #1080      +/-   ##
==========================================
- Coverage   61.09%   61.04%   -0.05%     
==========================================
  Files         264      264              
  Lines       26222    26225       +3     
==========================================
- Hits        16020    16009      -11     
- Misses       8524     8532       +8     
- Partials     1678     1684       +6     

Files with missing lines	Coverage Δ
keystore/keystore.go	77.38% <75.00%> (-2.87%)	⬇️

... and 9 files with indirect coverage changes

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