Fix parse_dataset error handling

Fix parse_dataset error handling

A mount failure was accidentally introduced by commit 0c1171d
which reworked the parse_dataset() function to read pool names
from devices.  The error case where a label is read from the
device but the pool name/value pair doesn't exist was not
handled properly.  In this case we should fall back to the
previous behavior.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1560