Improve error message when the upstream branch is wrong.

Improve error message when the upstream branch is wrong.

Otherwise, I get a stack trace, which is annoying. Here's an example:

$ git cl upload
Current branch help_text is up to date.

Failed to diff against upstream branch refs/heads/foo!

This branch probably doesn't exist anymore. To reset the
tracking branch, please run
    git branch --set-upstream help_text trunk
replacing trunk with origin/master or the relevant branch


R=dpranke@chromium.org
BUG=
TEST=


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=106175

[Dirk Pranke, 2011-10-18 21:03:35]

lgtm

[commit-bot: I haz the power, 2011-10-18 23:58:34]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/maruel@chromium.org/8349011/1

[commit-bot: I haz the power, 2011-10-19 00:04:37]

Change committed as 106175