sphinx: return error source as integer

[joostjager]

Returning as an integer allows a node to occur in the payment path multiple times.

[joostjager]

How is this better than just using the pubkey as is? If they appear multiple times in the path, then they'll be present multiple times in the payment path's circuit.

The sender doesn't know whether the failure happened on the first or the second visit of that node.

[joostjager]

@Roasbeef fixup pushed that adds return struct. I still don't think it is good, for the reasons pointed out above. But it is a small change and the most important thing is to expose the index one way or the other.

[joostjager]

@Roasbeef ptal