github:// urls - proper error message when a target repository or branch doesn't exist

[peteruhnak]

Currently when I type in incorrect name of repo or a branch I just get a weird error saying that a zip wasn't found.

This is very cryptic when one encounters it for the first time. Why it shows error only when attempted to load the zip, and not during the download? Why did the download failed silently?

Ideally it could also tell what part is incorrect, e.g. good repo, but branch doesn't exist… but that would be probably unnecessarily complex.

[ThierryGoubier]

The code for handling github:// urls is in Metacello, not FileTree. Error detection and analysis is dependent on what the github API returns

[ThierryGoubier]

I'll also rename the issue, because it has nothing to do with GitFileTree :)