Change generic NotFound to specific exception

Change generic NotFound to specific exception

1. Changed exception type from NotFound to specific NotFound
   exception for better readability.
   e.g. VolumeNotFound, SnapshotNotFound etc.

2. Error messages returned to the user are not consistent across
   all apis in case of all exceptions derived from NotFound exception.
   Instead of returning custom error messages return message
   defined in exception class itself.
   NOTE: Doesn't applied to 'HostBinaryNotFound' as host information to
         the tenant should not be leaked.

3. Added explanation to HTTPNotFound wherever appropriate.
   e.g. raise webob.exc.HTTPNotFound() to
        raise webob.exc.HTTPNotFound(explanation=err.msg)

Closes-Bug: #1459958
Change-Id: Ice2c375796fac7533d82125ef97288075fa68b85