Make encrypted "zfs mount -a" failures consistent

Currently, "zfs mount -a" will print a warning and fail to mount
any encrypted datasets that do not have a key loaded. This patch
makes the behavior of this failure consistent with other failure
modes ("zfs mount -a" will silently continue, explict "zfs mount"
will print a message and return an error code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #7382