R1:63be89ba00c9

Improve error message for error 2006

Summary:
See discussion here:

https://secure.phabricator.com/chatlog/channel/%23phabricator/?at=21186

Basically, MySQL usually raises a good error if we exceed "max_allowed_packet":

EXCEPTION: (AphrontQueryException) #1153: Got a packet bigger than 'max_allowed_packet' bytes

But sometimes it gives us a #2006 instead. This is documented, at least:

>"With some clients, you may also get a Lost connection to MySQL server during query error if the communication…