But in the cause of incremental positive improvement, here's a suggestion for a patch to the file that might help the next newbie resolve this without help:
Personally, I think that leaving the import bare and letting it fail is the right way. If you look at most python apps, they do not try to gracefully import things that are core dependencies and print special messages. We do that for things that are optional that we can work around if they're not present, but pyserial is not optional and thus I don't think we should be defensive about such things.
If others think this is really important, then that's fine, but IMHO, it's just not worth it. It's documented on the developers page as a requirement and was just a little less obvious because you're running out of the MacOS package.
--Dan