fix: error message for dangling reference index

[JeyJeyGao]

Test example:

notation sign docker.io/xxx/hello:v1 --plugin azure-kv --id https://xxx.vault.azure.net/certificates/xxx/xxx --plugin-config credential_type=azurecli -v
INFO The referrers tag schema is always attempted 
INFO Reference v1 resolved to manifest descriptor: {MediaType:application/vnd.docker.distribution.manifest.v2+json Digest:sha256:f54a58bc1aac5ea1a25d796ae155dc228b3f0e11d046ae276b39c4bf2f13d8c4 Size:525 URLs:[] Annotations:map[] Data:[] Platform:<nil> ArtifactType:} 
Warning: Always sign the artifact using digest(@sha256:...) rather than a tag(:v1) because tags are mutable and a tag reference can point to a different artifact than the one signed.
Warning: Removal of outdated referrers index from remote registry failed. Garbage collection may be required.
Successfully signed docker.io/mxd950630/hello@sha256:f54a58bc1aac5ea1a25d796ae155dc228b3f0e11d046ae276b39c4bf2f13d8c4

Resolves #400
Signed-off-by: Junjie Gao [email protected]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.17%. Comparing base (9ff1891) to head (a42ab40).
Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #402      +/-   ##
==========================================
+ Coverage   73.43%   80.17%   +6.74%     
==========================================
  Files          24       31       +7     
  Lines        2507     2174     -333     
==========================================
- Hits         1841     1743      -98     
+ Misses        526      302     -224     
+ Partials      140      129      -11     

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[Two-Hearts]

Just a note, this PR depends on PR #405.

[Two-Hearts]

LGTM