collapse all partition exceptions into DeadReferenceError

[Brian Warner]

At the moment, applications which want to send a message and ignore partitions must catch three separate exceptions:

• DeadReferenceError (which occurs when the connection is lost before they send the message)
• ConnectionLost (which occurs when the other end drops the connection while the message is outstanding)
• ConnectionDone (which occurs when we drop the connection while the message is outstanding, probably due to a duplicate connection, probably because the other side is the pairwise master and initiated a connection to us)

The connection exceptions should be caught internally and converted into a DeadReferenceError, so that applications only have to catch a single type.