Reset gem path after installing Overcommit

When running the Overcommit hook when Overcommit has not been installed,
the user is asked to install it. However, after installation you could
not load the `overcommit` gem since the gem path was already cached.

Fix this by resetting the gem path.

Change-Id: I868edd68ef4603610faf3d8f98f648f2691dfe38
Reviewed-on: http://gerrit.causes.com/35619
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>
Display better error when unable to install

When attempting to install Overcommit as part of running the hook, we
didn't give an informative error if for whatever reason this install
failed.

Improve the error message and change the exit status to follow the
`sysexits` standard.

Change-Id: I91fe79932a93cc9df5a7568fd223287c956e0f27
Reviewed-on: http://gerrit.causes.com/35620
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>