Check git remote exists

[dberenbaum]

Fixes iterative/dvclive#753.

Before this PR:

$ python train.py
WARNING:dvc_studio_client:Failed to post to Studio: {"detail":"Permission denied or repo not found, check that origin is a self-hosted gitlab repo"}
WARNING:dvclive:`post_to_studio` `start` failed.

After this PR:

$ python train.py
WARNING:dvc_studio_client:Couldn't find a valid Studio Repo URL.
You can try manually setting the environment variable `STUDIO_REPO_URL`.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4563f8b) 100.00% compared to head (a8f1f49) 100.00%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #125   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          292       297    +5     
  Branches        16        17    +1     
=========================================
+ Hits           292       297    +5     

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[dberenbaum]

Closing in favor of #126