fix: gracefully handle when git executable can't be found

[mndeveci]

Which issue(s) does this change fix?

#5983

Why is this change necessary?

When checking git executable, SAM CLI raises OSError which causes hard fail.

How does it address the issue?

This replaces OSError with a custom one which extends from UserError. This will display a better warning message rather than splitting the whole exception trace.

What side effects does this change have?

N/A

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.