Don't double wrap ServiceMayNotContinueExceptions

ServiceMayNotContinueException is an IOException, and should just be
re-raised in situations that need to throw a
ServiceMayNotContinueException.

Also, do not set the message on ServiceMayNotContinueException, since it
may be returned to the client.

Change-Id: I382be1ccb2b1dca50328951239ff27c20f79d0d1