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 packet is too large."

http://dev.mysql.com/doc/refman//5.5/en/packet-too-large.html

Try to improve the error message to point at this as a possible explanation.

Test Plan: Faked an error, had it throw, read exception message. See also chatlog.

Reviewers: btrahan, skrul

Reviewed By: skrul

CC: aran

Differential Revision: https://secure.phabricator.com/D2923