Return meaningful error message on pool creation error

Instead of returning exception specific to haproxy (agent-based),
return more generic BackendNotFound exception.
It also could be used later when binding to devices will be
introduced. That exception will indicate scheduling failure, e.g.
inability to find appropriate backend for the resource.
Resource (pool) is then marked with ERROR state with corresponding
error description.

Change-Id: Ic18ff20102b4bb2b97e7b186fcf797133bd3ba3d
Closes-Bug: #1295214
Merge "Return meaningful error message on pool creation error"