Bug 8050: Die if package building fails

Bug 8050: Die if package building fails

The debian/build-git-snapshot script should not return success if an
error occurred.

Note that sys_command_output won't raise an error if something fails.

Signed-off-by: Robin Sheat <robin@catalyst.net.nz>
Made a minor change to print the command, add a newline.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
(cherry picked from commit 2e1c031ea0a779a6a4f9b58bf1b15db4b9056fd4)
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>