feat(actions): improve error message for wrong actions

[linrongbin16]

This PR improves the error message when executing the "Open URL in browser" action, thus help users debug.

Close #259

Test Platforms

• windows
• macOS
• linux

Test Hosts

• Test on github.com.
• Test on gitlab.com.
• Test on bitbucket.org.
• Test on codeberg.org.
• Test on git.samba.org.

Test Functions

• Use GitLink(!) to copy git link (or open in browser).
• Use GitLink(!) blame to copy the /blame link (or open in browser).
• Use GitLink(!) default_branch to open the /main//master link in browser (or open in browser).
• Use GitLink(!) current_branch to open the current branch link in browser (or open in browser).
• Copy git link in a symlink directory of git repo.
• Copy git link in an un-pushed git branch, and receive an expected error.
• Copy git link in a pushed git branch but edited file, and receive a warning says the git link could be wrong.
• Copy git link with 'file' and 'rev' parameters.

[codecov]

Codecov Report

Attention: Patch coverage is 68.75000% with 10 lines in your changes missing coverage. Please review.

Project coverage is 72.95%. Comparing base (296ad98) to head (5b4d420).

Files with missing lines	Patch %	Lines
lua/gitlinker/actions.lua	68.75%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #261      +/-   ##
==========================================
+ Coverage   70.82%   72.95%   +2.13%     
==========================================
  Files          10       10              
  Lines         963      991      +28     
==========================================
+ Hits          682      723      +41     
+ Misses        281      268      -13     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.