f reword error message

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -1574,7 +1574,9 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 				Ok(funding_msg) => {
 					(chan, funding_msg)
 				},
-				Err(_) => { return Err(APIError::ChannelUnavailable { err: "Error deriving keys - either our RNG or our counterparty's RNG is broken".to_owned() }) },
+				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()
+				}) },
 			}
 		};