Application errors should be DNS responses

[huitema]

The code currently supports three application errors that are not defined in the draft:

#define QUICDOQ_ERROR_RESPONSE_TOO_LONG 0x202
#define QUICDOQ_ERROR_RESPONSE_TIME_OUT 0x203
#define QUICDOQ_ERROR_QUERY_TOO_LONG 0x204

This is in contradiction with the general consensus that errors should generally be conveyed as DNS responses, not through QUIC codes. QUIC error codes should only be issued when the transport itself is lost, for example when there is not enough resource to send a message.