Error message missing information

I was trying to push to a repo where I didn't have write access and got a 403 error. The error message in GitUp only says "unexpected HTTP status code: 403". When using git from the shell, it also shows "remote: Write access to repository not granted."

Obviously that would be very valuable information to include in the error popup in gitup as well.

I'd have to see if we actually know that information in the API, but good suggestion.