More helpful message if --repo_path doesn't exist

[klieret]

Closes #763
Also see #761

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 74.24%. Comparing base (55fec07) to head (09352e0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sweagent/environment/utils.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #764      +/-   ##
==========================================
- Coverage   74.30%   74.24%   -0.06%     
==========================================
  Files          27       27              
  Lines        3713     3716       +3     
==========================================
  Hits         2759     2759              
- Misses        954      957       +3     

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