Introduce UnsupportedOperation EC2 error code

'UnsupportedOperation' ec2_code attribute was added to following
exceptions that are OpenStack specific and don't have mapping to EC2
error codes because EC2 simply doesn't encounter these problems at all.

 * CannotDisassociateAutoAssignedFloatingIP
 * FloatingIpAssociated
 * FloatingIpNotFound
 * NoFloatingIpInterface
 * NoMoreFixedIps
 * InvalidVolume

Catch these direcly in Executor instead of translating them to useless
EC2APIError. This way, user will also get meaningful error messages.

Implements: blueprint ec2-error-codes

Change-Id: I62ea97658cee85e5285f55cba600a08d01fe7cdd
Merge "Introduce UnsupportedOperation EC2 error code"