* `basic_parser` now uses pure virtual member functions instead
of the curiously recurring template pattern.
Actions Required:
* Change uses of the `basic_parser` type to omit the `Derived`
template parameter
* Classes derived from `basic_parser` no longer need to friend
the base.
* Virtual functions in the derived class may be marked `override`.
fix#949
* New error codes are introduced for WebSocket failures
* More verbose messages for error codes
* Error codes are mapped to conditions for ease of testing
* error::failed and error::handshake_failed are deprecated (don't use)
Actions Required:
* Code which explicitly compares error_code values against the
constant `websocket::error::handshake_failed` should compare
against `websocket::condition::handshake_failed` instead.
* Code which explicitly compares error_code values against the
constant `websocket::error::failed` should compare
against `websocket::condition::protocol_violation` instead.
This fixes a problem where an assert is generated or an
error is ignored when an invalid deflate stream is produced
after appending the final empty deflate block.
