support 62-bit error codes

[kazuho]

The error code space of quicly is based on the old draft that used 16-bit values.

Specifically, errors are signaled using int, which is divide into:

range	usage
0x000000??	picotls (TLS alerts from host)
0x000001??	picotls (TLS alerts from peer)
0x000002??	picotls (internal errors)
0x0000ff??	quicly internal error codes1
0x0002????	QUIC transport error codes
0x0003????	QUIC application error codes

We eventually need to switch to a scheme that supports full 62-bit range for QUIC transport and applications.

[1] Even though the value space is internal, some are used to signal transport errors. In such case, the values are converted to transport error codes upon emission.