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

@@ -634,11 +634,7 @@ pub fn do_test(data: &[u8], logger: &Arc<dyn Logger>) {
 					if let Err(e) = channelmanager.funding_transaction_generated(&funding_generation.0, &funding_generation.1, tx.clone()) {
 						// 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");
-							}
-						} else { panic!(); }
+						if let APIError::ChannelUnavailable { .. } = e { } else { panic!(); }
 					}
 					pending_funding_signatures.insert(funding_output, tx);
 				}

lightning/src/ln/channelmanager.rs

@@ -2691,7 +2691,7 @@ where
 					(chan, funding_msg)
 				},
 				Err(_) => { return Err(APIError::ChannelUnavailable {
-					err: "Error deriving keys or signing initial commitment transactions - either our RNG or our counterparty's RNG is broken or the Signer refused to sign".to_owned()
+					err: "Signer refused to sign the initial commitment transaction".to_owned()
 				}) },
 			}
 		};