fix: plugin error message

[JeyJeyGao]

Examples:

When putting an arbitrary file into the plugin folder:

NAME                   DESCRIPTION   VERSION   CAPABILITIES   ERROR                                                                                                                                                                                                                              
azure-trustedsigning                           []             fork/exec /home/jj/.config/notation/plugins/azure-trustedsigning/notation-azure-trustedsigning: exec format error. Please ensure that the plugin executable file is compatible with linux/amd64

When the plugin name is not followed the plugin spec:

NAME                   DESCRIPTION   VERSION   CAPABILITIES   ERROR                                                                                                                                                                                                                                                                                                                        
azure-trustedsigning                           []             stat /home/jj/.config/notation/plugins/azure-trustedsigning/notation-azure-trustedsigning: no such file or directory. Each plugin executable must be located under $PLUGIN_DIRECTORY/{plugin-name} directory, with executable named as notation-{plugin-name}

When the notation plugin executable file is not executable:

NAME                   DESCRIPTION   VERSION   CAPABILITIES   ERROR                                                                                                               
azure-trustedsigning                           []             fork/exec /home/jj/.config/notation/plugins/azure-trustedsigning/notation-azure-trustedsigning: permission denied

When using an AMR64 arch of plugin file on an AMD64 machine:

NAME                   DESCRIPTION   VERSION   CAPABILITIES   ERROR                                                                                                                                                                                                                              
azure-trustedsigning                           []             fork/exec /home/jj/.config/notation/plugins/azure-trustedsigning/notation-azure-trustedsigning: exec format error. Please ensure that the plugin executable file is compatible with linux/amd64

Resolves #704

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.52%. Comparing base (167bfd4) to head (7f23e76).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/notation/plugin/list.go	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1217      +/-   ##
==========================================
+ Coverage   76.33%   76.52%   +0.19%     
==========================================
  Files          67       67              
  Lines        3697     3710      +13     
==========================================
+ Hits         2822     2839      +17     
+ Misses        675      672       -3     
+ Partials      200      199       -1     

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