commit: Alert the user when "git commit" fails

Show an informative dialog box when "git commit" returns a
non-zero exit status.  This prevents git-cola from failing
silently when a pre-commit hook aborts the commit.

Closes #160

Reported-by: Kerrick Staley <mail@kerrickstaley.com>
Signed-off-by: David Aguilar <davvid@gmail.com>