Improve error message in branch existence check

[adamzip]

#6141

[Copilot]

Pull request overview

Updates DARC’s set-repository-policies operation to perform an explicit branch existence check via IRemote.BranchExistsAsync, aiming to produce clearer failures when branch verification can’t be performed.

Changes:

• Replaced UxHelpers.VerifyAndConfirmBranchExistsAsync(...) with a direct BranchExistsAsync call.
• Added exception handling and logging around the branch existence verification path.

[premun]

@adamzip should we still narrow down the exception we're catching? Seems like we might be masking other problems - like if GitHub 500s the user will think it's a PAT problem and will go and regenerate it only to find out it didn't help anything

[adamzip]

@adamzip should we still narrow down the exception we're catching? Seems like we might be masking other problems - like if GitHub 500s the user will think it's a PAT problem and will go and regenerate it only to find out it didn't help anything

I wanted to avoid parsing the error message of the HttpRequestException if possible, so ideally the error message would just inform the user what it was trying to do, and then print whatever error message came out.
How about this then:

Error: Darc attempted to make a remote git requets on your behalf at {repo}@{branch}, using the configured credentials. The request failed with the following error message: [...]

[premun]

That's maybe a better error message but do we need to parse the error message?

• Isn't it better to catch HttpRequestException then?
• Doesn't HttpRequestException have StatusCode that you can filter based on? Maybe that's enough to tell and let the rest go through?

[adamzip]

That's maybe a better error message but do we need to parse the error message?

• Isn't it better to catch HttpRequestException then?
• Doesn't HttpRequestException have StatusCode that you can filter based on? Maybe that's enough to tell and let the rest go through?

I implemented your suggestion, feel free to change the message wording