This patch removes the node from the RPC error constructurs
CurlLayerError and OfflineNodeError. The rationale is that we anyway
return tuples (node, result), and removing this duplication allows
simplified signatures/calls in the execution of RPC calls.

Note: this was sent before as well, resending on top of current codebase.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>