Correct funding_transaction_generated err msg and fix fuzz check

This fixes new errors in `full_stack_target` pointed out by
Chaincode's generous fuzzing infrastructure. Specifically, there's
no reason to check the error message in the
`funding_transaction_generated` return value - it can only return
a failure if the channel has closed since the funding transaction
was generated (which is fine) or if the signer refuses to sign
(which can't happen in fuzzing).

Author: TheBlueMatt

fuzz/src/full_stack.rs

@@ -635,9 +635,10 @@ pub fn do_test(data: &[u8], logger: &Arc<dyn Logger>) {
 						// It's possible the channel has been closed in the mean time, but any other
 						// failure may be a bug.
 						if let APIError::ChannelUnavailable { err } = e {
-							if !err.starts_with("Can't find a peer matching the passed counterparty node_id ") {
-								assert_eq!(err, "No such channel");
-							}
+							assert!(
+								err.starts_with("Can't find a peer matching the passed counterparty node_id ") ||
+								(err.starts_with("Channel with id ") && err.contains(" not found for the passed counterparty node_id "))
+							);
 						} else { panic!(); }
 					}
 					pending_funding_signatures.insert(funding_output, tx);