Clarify error message when importing web app files

[isc-pbarton]

Some tweaks based on @cambot's comments on #839

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 49.78%. Comparing base (093ec2e) to head (cfe18c3).

Files with missing lines	Patch %	Lines
cls/SourceControl/Git/Utils.cls	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #842   +/-   ##
=======================================
  Coverage   49.78%   49.78%           
=======================================
  Files          24       24           
  Lines        3274     3274           
=======================================
  Hits         1630     1630           
  Misses       1644     1644           

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