When the Git plugin fails, it gives the error "Nothing to do." This doesn't help diagnose the problem so much, particularly if it's something trivial that the user did (like put in an incorrect branch).
The error message could help users for the most trivial cases, like "Couldn't find anything to download from the Git repository. Did you get the URL or branch name right?" Obviously, this would be changed to list the cases where "Nothing to do" might be thrown