Added a convention that sentinel errors go near the top of the file, and applied it

internal/websocket/client.go

@@ -24,6 +24,10 @@ const clientSendBuffer = 256
 // cut short and logged as truncated.
 const logMessageLimit = 1 << 10
 
+// errUnsupportedData is returned by readLoop (and a tcpConn's Write) for a
+// binary message.
+var errUnsupportedData = errors.New("websocket: binary frames are not supported")
+
 // msgConn is the connection a client reads and writes messages on: a
 // *websocket.Conn, or a tcpConn for plain TCP clients (see tcp.go).
 type msgConn interface {
@@ -51,10 +55,6 @@ type client struct {
 	closeOnce sync.Once
 }
 
-// errUnsupportedData is returned by readLoop (and a tcpConn's Write) for a
-// binary message.
-var errUnsupportedData = errors.New("websocket: binary frames are not supported")
-
 // newClient creates a client for conn. cancel cancels the context the client
 // runs with; the client calls it when it finishes or is closed. The client's
 // messages are limited by websocket.message_rate_limit if it is enabled.