Handle FloatingIpPoolNotFound exception in floating ip creation

If an invalid pool is specified when creating a floating ip
with neutron as the backend a FloatingIpPoolNotFound exception
can be raised. This change handles the exception at the API level.

Change-Id: Iac46d86a11f2c1da0ed7e7861cdb87fd4d4b4c6c
Closes-Bug: 1352141
Merge "Handle FloatingIpPoolNotFound exception in floating ip creation"