update: updated plugin list command

[patrickzheng200]

This PR mainly fixed the plugin list command.
Previously, if there's an error on getting a plugin, only the error will be printed out.
In this PR, on error, the pluginName will also be printed out along with the error.
(This PR also includes some of the minor cleanups of notation CLI.)

Signed-off-by: Patrick Zheng [email protected]

[codecov-commenter]

Codecov Report

Merging #461 (9fb2e7f) into main (86287b5) will decrease coverage by 0.28%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #461      +/-   ##
==========================================
- Coverage   32.48%   32.19%   -0.29%     
==========================================
  Files          25       25              
  Lines        1250     1261      +11     
==========================================
  Hits          406      406              
- Misses        832      843      +11     
  Partials       12       12              

Impacted Files	Coverage Δ
cmd/notation/cert/generateTest.go	16.52% <0.00%> (-0.15%)	⬇️
cmd/notation/cert/list.go	13.55% <0.00%> (-0.24%)	⬇️
cmd/notation/key.go	24.75% <0.00%> (ø)
cmd/notation/plugin.go	0.00% <0.00%> (ø)
cmd/notation/verify.go	22.07% <0.00%> (ø)

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[shizhMSFT]

LGTM

[priteshbandi]

LGTM with suggestions

[shizhMSFT]

LGTM