mgr / volumes: use negative error codes everywhere

mgr / volumes: use negative error codes everywhere

cephfs python binding returns positive error code. mgr/volumes
incorrectly does error code checks assuming the error codes to
be negative.

this was not an issue till now since mgr/volumes mostly does a
`raise VolumeException()` for the most part followed by the exception
being displayed to the operator (one exception is catching cephfs
ObjectNotFound error, in which case -errno.ENOENT is returned
(and checked whereever required)).

Signed-off-by: Venky Shankar <vshankar@redhat.com>