Simplify RPC error cases

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>