GetSignaturesProtectedBranch Does Not Return ErrBranchNotProtected When it Could #2821

When calling the GitHub API endpoint https://api.github.com/repos/OWNER/REPO/branches/BRANCH/protection/required_signatures (documentation), if the branch does not have any protection rules, the GitHub API will reply with a HTTP 404 Not Found code and the following body:

{
  "message": "Branch not protected",
  "documentation_url": "https://docs.github.com/rest/branches/branch-protection#get-commit-signature-protection"
}

Other functions in go-github that may receive this kind of response usually manage it the following way:

However, GetSignaturesProtectedBranch does not:

I think this function should have the same behaviour, allowing callers to handle this case the same way as when calling say GetBranchProtection. Note that function isBranchNotProtected sounds like it would work without changes:

I am happy to provide a Pull Request (should be trivial) for this.