[REQ] Better error reporting when git rebase cannot rebase

[Jake Goulding <goulding@vivisimo.com>]

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