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

src/infrastructure/storage/connection/mysql/AphrontMySQLDatabaseConnectionBase.php

@@ -217,8 +217,11 @@ abstract class AphrontMySQLDatabaseConnectionBase
 
     switch ($errno) {
       case 2013: // Connection Dropped
-      case 2006: // Gone Away
         throw new AphrontQueryConnectionLostException($exmsg);
+      case 2006: // Gone Away
+        $more = "This error may occur if your MySQL 'wait_timeout' ".
+                "or 'max_allowed_packet' configuration values are set too low.";
+        throw new AphrontQueryConnectionLostException("{$exmsg}\n\n{$more}");
       case 1213: // Deadlock
       case 1205: // Lock wait timeout exceeded
         throw new AphrontQueryDeadlockException($exmsg);