pwclient: exit on first patch which fails to apply

When run with more than one patch ID, the 'apply' and 'git-am' commands
should not continue to process other patches if an earlier one failed.
We should stop so the user can address the situation.

Future work: it'd be nice to just pipe all the patches at once to
git-am, so that git's nice handling of fixup-and-continue workflow can
be used.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>