fix: rename error for verifier plugins to be more generic

[akashsinghal]

Description

What this PR does / why we need it:

Currently, external verifier plugin failures are wrapped in a single error code that's specific to signature failures. this is confusing for non signature artifacts such as vuln scans and SBOMs. This PR makes the error code more generic. The error code contains the plugin name which provides enough information to determine if it's a signature or not.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):

Corresponding doc change here: ratify-project/ratify-web#20

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Helm Chart Change (any edit/addition/update that is necessary for changes merged to the main branch)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also list any relevant details for your test configuration

• Test A
• Test B

Checklist:

• Does the affected code have corresponding tests?
• Are the changes documented, not just with inline documentation, but also with conceptual documentation such as an overview of a new feature, or task-based documentation like a tutorial? Consider if this change should be announced on your project blog.
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have appropriate license header?

Post Merge Requirements

• MAINTAINERS: manually trigger the "Publish Package" workflow after merging any PR that indicates Helm Chart Change

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f5eaa99) 52.30% compared to head (b8cec9b) 52.30%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1129   +/-   ##
=======================================
  Coverage   52.30%   52.30%           
=======================================
  Files         101      101           
  Lines        6305     6305           
=======================================
  Hits         3298     3298           
  Misses       2688     2688           
  Partials      319      319           

Files	Coverage Δ
pkg/verifier/notation/notation.go	86.04% <100.00%> (ø)
pkg/verifier/plugin/plugin.go	70.83% <0.00%> (ø)

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[susanshi]

LGTM. thanks!