bash_completion: replace exit with return

Not sure what was I thinking about when I added all those 'exit'
statements, but the result is exiting from current shell when trying
to use completion in specific places, for example

	vzctl snapshot XXX --id <Tab>

This also happens then trying to complete CTIDs, arguments of --ipdel
and --id/--uuid options by a non-root user.

Fix by replacing exit with return everywhere.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>