Refactor WebSocket errors (API Change):

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.

test/beast/websocket/close.cpp

@@ -108,7 +108,7 @@ public:
             catch(system_error const& se)
             {
                 BEAST_EXPECTS(
-                    se.code() == error::failed,
+                    se.code() == error::bad_masked_frame,
                     se.code().message());
             }
         }
@@ -128,7 +128,7 @@ public:
             catch(system_error const& se)
             {
                 BEAST_EXPECTS(
-                    se.code() == error::failed,
+                    se.code() == error::bad_close_size,
                     se.code().message());
             }
         }
@@ -292,7 +292,7 @@ public:
             ws.async_read(b,
                 [&](error_code ec, std::size_t)
                 {
-                    if(ec != error::failed)
+                    if(ec != error::bad_control_fragment)
                         BOOST_THROW_EXCEPTION(
                             system_error{ec});
                     BEAST_EXPECT(++count == 1);