Refactor WebSocket error codes (API Change):

fix #325

This removes unused and misleading handshake error codes.
All semantic handshake failures are now reported using
the same code, error::handshake_failed. Errors originating
from stream operations still use the underlying stream
error codes (for example: boost::asio::error::connection_reset).

test/websocket/stream.cpp

@@ -1107,7 +1107,7 @@ public:
                 }
                 catch(system_error const& se)
                 {
-                    BEAST_EXPECT(se.code() == error::response_failed);
+                    BEAST_EXPECT(se.code() == error::handshake_failed);
                 }
             };
         // wrong HTTP version