rpcServer: ensure we don't get blocked on bidirectional payment errors
rpcServer: ensure we don't get blocked on bidirectional payment errors

This commit fixes a potential issue in the bidirectional sendPayment
case, where multiple goroutines could be sending on an errChan with
buffer 1. Instead we select on default as well, as it is enough to
handle the first error that was received.

LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd