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

`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](https://github.com/Azure/azure-cli-dev-tools/blob/dev/azdev/operations/code_gen.py#L140-L162) logic should probably print a warning instead of fail, if not in the azure-cli or azure-cli-extensions repos.