[REQ] Better error reporting when git rebase cannot rebase

Right now, if you have modified files in the working directory and attempt to do a git rebase, you will see messages like:

$ git rebase origin/master
Makefile: needs update

However, it isn't always clear that that means the rebase *did not work*. This has bitten a few of my developers a few times when they think that a rebase worked and that they need to perform some sort of "update" to the files in question.

I would suggest making this more explicit:

$ git rebase origin/master
* error: rebasing branch master on branch origin/master failed
Makefile: uncommitted changes would be overwritten

Thanks!

-Jake