Fix unexpected error message when git is not installed

[jon-fearer]

resolves #9537

Problem

The dbt debug command returns an unexpected error message when git is not installed

Solution

Update the test_git method to handle the CommandResultError exception that is thrown when git is not installed.

Output when git is installed:

Output when git is not installed (updated behavior):

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

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[jon-fearer]

Whoops.. forgot to sign the CLA before submitting this. I have now signed it.

[jon-fearer]

@dbeatty10 @peterallenwebb Let me know if anything else is needed from me here. Thank you