Exit with non-zero status when no examples run

For our purposes, there isn't really any good reason to return an exit
code of zero when no specs are run (it almost invariably means something
went wrong).

This changes the behaviour of Buffet to return a non-zero status code in
the event no specs are run.

Change-Id: I0394e8c40fd60ce3d577551e8d631d046bbf5f6c
Reviewed-on: https://gerrit.causes.com/19828
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>