don't send application-level errors before completion of the handshake

internal/qerr/quic_error.go

@@ -16,6 +16,10 @@ type QuicError struct {
 
 var _ net.Error = &QuicError{}
 
+// UserCanceledError is used if the application closes the connection
+// before the handshake completes.
+var UserCanceledError = &QuicError{ErrorCode: 0x15a}
+
 // Error creates a new QuicError instance
 func Error(errorCode ErrorCode, errorMessage string) *QuicError {
 	return &QuicError{