Improve <NOTOPEN> error reporting (#462)

[isc-pbarton]

Description

Resolves #462
Resolves #981

NOTOPEN errors running git commands mask an underlying OS error. We now report that OS error and suggest the root cause in the error message: either the git executable cannot be found, or we don't have permissions to run it.

Testing

Unit tests provide good coverage on Unix. I did some manual testing on Windows:

• Git executable is not on the path and not configured on the settings page. The Settings page shows an error message like "git was not found. Make sure git is installed and on the IRIS user's PATH, or set an absolute path to the git executable on the Settings page." Running a git command like do ##class(SourceControl.Git.Utils).Clone("blah") throws the same error.
• Git executable is configured to be a file that can't actually be executed, like an empty file. Both cases display a different error message "git could not be launched (<193> %1 is not a valid Win32 application.)"

Checklist

• This branch has the latest changes from the main branch rebased or merged.
• Web UI has been built (any changes in git-webui/src have matching changes in git-webui/release)
• CHANGELOG.md entry added if appropriate.
• [N/A] Documentation has been/will be updated

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.26087% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.38%. Comparing base (c12184c) to head (4f1634f).

Files with missing lines	Patch %	Lines
cls/SourceControl/Git/Utils.cls	78.26%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #985      +/-   ##
==========================================
+ Coverage   49.14%   49.38%   +0.24%     
==========================================
  Files          22       22              
  Lines        3315     3335      +20     
==========================================
+ Hits         1629     1647      +18     
- Misses       1686     1688       +2     

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