feat: add debug information to cert validation error

[asraa]

Signed-off-by: Asra Ali asraa@google.com

Summary

• Adds debug information to the cert validation error for issuer/subject.

I found that when using these flags, it was difficult to know the expected calls. I think allowing users to see the failing information (did I get the issuer right?) would help with using the CLI.

Displays like:

$ ./cosign verify-blob README.md --bundle bundle1.sig  --certificate-identity asraa@google.com --certificate-oidc-issuer-regexp .*google.*
Error: verifying blob [README.md]: none of the expected identities matched what was in the certificate, got subjects [asraa@google.com] with issuer https://github.com/login/oauth
main.go:62: error during command execution: verifying blob [README.md]: none of the expected identities matched what was in the certificate, got subjects [asraa@google.com] with issuer https://github.com/login/oauth

Release Note

Documentation

[codecov-commenter]

Codecov Report

Merging #2579 (4dc63f2) into main (fa6c58f) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2579      +/-   ##
==========================================
+ Coverage   30.02%   30.05%   +0.02%     
==========================================
  Files         141      141              
  Lines        8659     8662       +3     
==========================================
+ Hits         2600     2603       +3     
  Misses       5678     5678              
  Partials      381      381              

Impacted Files	Coverage Δ
pkg/cosign/verify.go	39.41% <100.00%> (+0.21%)	⬆️

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