cli-plugins/manager: use stdlib errors, and minor cleanup

[thaJeztah]

- What I did

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 59.26%. Comparing base (e201b4e) to head (8fc0c74).
Report is 34 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5903   +/-   ##
=======================================
  Coverage   59.26%   59.26%           
=======================================
  Files         357      357           
  Lines       29771    29771           
=======================================
  Hits        17645    17645           
  Misses      11153    11153           
  Partials      973      973           

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[thaJeztah]

Hm.. did I break something? Last PR was happy :thinking_face:

=== Failed
=== FAIL: e2e/plugin TestInstallWithContentTrust (4.27s)
    trust_test.go:40: assertion failed: 
        Command:  docker plugin push registry:5000/plugin-content-trust
        ExitCode: 1
        Error:    exit status 1
        Stdout:   The push refers to repository [registry:5000/plugin-content-trust]
        073f57518a8b: Preparing
        dae68ee65fc3: Preparing
        51df3a2ed0d0: Preparing
        073f57518a8b: Preparing
        dae68ee65fc3: Preparing
        51df3a2ed0d0: Preparing
        
        Stderr:   error pushing plugin: failed to do request: Head "https://registry:5000/v2/plugin-content-trust/blobs/sha256:dae68ee65fc39a8460c91e29347bfaf09e2c71df6329aa37a0946cfd6b8492a6": http: server gave HTTP response to HTTPS client
        
        
        Failures:
        ExitCode was 1 expected 0
        Expected stdout to contain "Signing and pushing trust metadata"
        Expected no error