This patch adds a simple failure reporting tool, similar to bdev's
_ThrowError. In backend, we move towards the new-style RPC results (of
type (status, payload)) and thus functions which use this style can very
easily log and return the error message using this new function.

The exception is declared here and not in errors.py since it's local to
the node-daemon/backend combination.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>