Adds Serf::Errors::LoadFailure in SerfUp.

Details:
* Error messages were ambiguous and hard to track down when there
  are errors in the initial start up loading of Serfs.
* We add a LoadFailure error class, which nests the original
  exception. But it gives a message (hint) to where the
  error occurred. If it was in reading the serfup file, or
  in the construction in the serf blocks.
* Updates the serf specs; adds test data serf files.