geli attach: Fix exit codes and errors not being printed after r335673

Now that multiple providers can be attached at once, exit codes and
error messages must be handled correctly if there are failures in on
any of the providers.

Reported by: asomers (Kyua test failures via continuous integration)
Reviewed by: asomers
Approved by: allanjude
Differential Revision: https://reviews.freebsd.org/D16386