Allow to delete a volume in error_extending status

Allow to delete a volume in error_extending status

We must be able to remove "error_extending" volume, because we
cannot retrieve it without changing database status.
If trying to remove the volume, now it fails like the following:

$ cinder delete vol-test01
ERROR: Invalid volume: Volume status must be available or error,
but current status is: error_extending
$

"error_extending" also would be error status, so it is good to
remove the volume.

Fixes bug #1214228

Change-Id: I861e7af75f2c50d4e03ef3680dccde73078b4888