Swallow transient packet parse errors

[algesten]

Summary

• hide transient packet parse errors from generated docs and move them to the end of Error
• swallow ParseIncomplete, ParseError, and TooManyRecords from handle_packet
• update edge/cookie tests to assert malformed datagrams are discarded

The idea here is that UDP packets could be mangled in route between client and server, or someone could spray garbage and/or intercepted packets at the server. Failed parsing is not a cause to throw out the connection.

This changes so all Error raised by dimpl publicly mean "throw away this connection"