rfqmsg: add reject-with-custom-message utility

ErrRejectWithCustomMsg constructs a RejectErr with error code 0 and the
specified error message.

Author: jtobin

rfqmsg/reject.go

@@ -92,6 +92,15 @@ var (
 	}
 )
 
+// ErrRejectWithCustomMsg produces the "unknown" error code, but pairs
+// it with a custom error message.
+func ErrRejectWithCustomMsg(msg string) RejectErr {
+	return RejectErr{
+		Code: 0,
+		Msg:  msg,
+	}
+}
+
 const (
 	// latestRejectVersion is the latest supported reject wire message data
 	// field version.