New class RPC::Status.

Needed for:
  https://ripplelabs.atlassian.net/browse/RIPD-97
  https://ripplelabs.atlassian.net/browse/RIPD-92
  https://ripplelabs.atlassian.net/browse/RIPD-98
  https://ripplelabs.atlassian.net/browse/RIPD-439

See design documents: 

  General:  https://docs.google.com/a/ripple.com/document/d/1_3AzruLKOUDhYaXoCNw0BzyfWzHIrDh5gAYZR7jWpEM/edit#heading=h.dbnm2m5xb68b

  Status:
https://docs.google.com/a/ripple.com/document/d/1KuqoBby20Ck7u2AB8TxBmQiBzGHNy5Mx3eFVNebze1k/edit#heading=h.jsddwcoor9j

Reviewed-by: @nbougalis @jjuran