Divergent behavior for over-long length on begin cells

When we get some other kind of error when parsing a begin cell, we send back an end cell. But if the error is an over-long length, we do nothing and just mark the connection. That's probably not right; if it is, it should be commented.

I needed to add extra code to keep the current behavior in the ipv6_exits branch; it would be neat to rip that out.