Add some error codes

[martinthomson]

In #467, we removed a bunch of error codes. This tracks whether we want to restore any of those.

The following might be invalid: HANDSHAKE_TIMEOUT, NETWORK_IDLE_TIMEOUT, PUBLIC_RESET_RECEIVED, TOO_MANY_RTOS.

The following probably depend on a resolution to #608: UNENCRYPTED_STREAM_DATA, INVALID_VERSION_NEGOTIATION_PACKET, ADDRESS_VALIDATION_FAILURE

A code for closing the crypto stream (or critical streams, if we ever have that concept) might depend on #485.

For PAYLOAD_MISSING we already have generic codes that could be used, we might also not bother to decrypt a packet that is only long enough to hold the authentication tag and so the error might never be detected.

When adding these back, we should describe the conditions under which they are generated.

[ianswett]

Will re-add connection close errors as necessary in the text.