fix: update error in SignedEntity to be more descriptive

[vishal-chdhry]

Summary

Closes #2930

The exact error described in the issue cannot happen anymore as that behavior was changed in v2.1.0.

PR #2959 updated the old behaviour and added an if case where the error message is used in an if statement. The issue with this is that the error value is lost when ErrEntityNotFound error is created. This creates a problem where the error in SignedEntity are not descriptive enough when remoteGet function fails.

This PR will:

• Add a function called NewEntityNotFoundError which uses the error received by remoteGet to create a new error.
• Add a function called IsEntityNotFoundError which is used in the if statement that checks if the error message contains an ErrEntityNotFound string.

Release Note

Updated error in SignedEntity to be more descriptive

cc @znewman01

[codecov]

Codecov Report

Merging #3233 (207124c) into main (0d48aa1) will decrease coverage by 0.02%.
Report is 2 commits behind head on main.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #3233      +/-   ##
==========================================
- Coverage   30.37%   30.35%   -0.02%     
==========================================
  Files         155      155              
  Lines        9828     9834       +6     
==========================================
  Hits         2985     2985              
- Misses       6397     6403       +6     
  Partials      446      446              

Files Changed	Coverage Δ
cmd/cosign/cli/sign/sign.go	14.25% <0.00%> (ø)
pkg/oci/remote/remote.go	36.77% <0.00%> (-1.49%)	⬇️

[znewman01]

Thanks for this change! Overall looks good, I have a couple of Go-specific requests.