If a tsugi launch fails, or the session value is lost, or the session expires, or if there is a browser signature mismatch, Tsugi displays the attached message.
You can override the destination with a $CFG variable - if yuo already have your own launch fail processing that wil be unchanges - but the default behavior will be changed according to this issue:
The short version is that the LTI launch return URL is a better place to go.
Chances are good you will ever notice this :)
/Chuck