azdev extension create fails if repo doesn't have CODEOWNERS file

[colbylwilliams]

azdev extension create requires the repo has a CODEOWNERS file or it fails with an error.

New extension in private repos (not the azure-cli-extensions repo) likely won't have a CODEOWNERS file, so the code generation logic should probably print a warning instead of fail, if not in the azure-cli or azure-cli-extensions repos.

[laurawalker]

Commenting to add that this is still an issue (just hit this myself).