Improve two error handling behaviors in arc upgrade

Summary:
Fixes T9222. Two issues here:

• First, we currently continue on error. Throw instead. I just swapped us from "phutil_passthru()" to "execx()" since I don't think printing out the "pulling from remote..." status messages is very important, and this makes it easier to raise a useful exception.
• Second, if you have a dirty working copy we currently may try to do some sort of silly stuff which won't work, like prompt you to amend changes. Instead, do a slightly lower-level check and just bail.

Test Plan:

• Ran arc upgrade with a dirty working copy and got a tailored, useful error.
• Ran arc upgrade with an artificially bad git pull command, got a failure with a specific error message.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9222

Differential Revision: https://secure.phabricator.com/D14317