gen: move an error check to catch all instances of failure

`m` might not be set on the optional set as well, so move this check
down so that we now encompass both codepaths

tools/generate-wire.py

@@ -1252,15 +1252,15 @@ def find_message_with_option(messages, optional_messages, name, option):
             # eg commit_sig,0,channel-id,8 OR
             #    commit_sig,0,channel-id,u64
             m = find_message(messages + subtypes, parts[0])
-            if m is None:
-                raise ValueError('Unknown message or subtype {}'.format(parts[0]))
         elif len(parts) == 5:
             # eg.
             # channel_reestablish,48,your_last_per_commitment_secret,32,option209
             m = find_message_with_option(messages, messages_with_option, parts[0], parts[4])
         else:
             raise ValueError('Line {} malformed'.format(line.rstrip()))
 
+        if m is None:
+            raise ValueError('Unknown message or subtype {}'.format(parts[0]))
         f = Field(m.name, parts[2], parts[3], comments, prevfield, includes)
         m.addField(f)
         # If it used prevfield as lenvar, keep that for next