Provide specific error message strings for two pool_ctr() failure cases
that currently give just "Unknown error".

Reference: test_two_pools_pointing_to_the_same_metadata_fails and
test_different_pool_cant_replace_pool in thinp-test-suite.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>