I have this code :

// this will provoke a fatal error because "Grap" without last "h"
$graph = new CanvasGrap(300,100);
if ( is_wp_error($graph) ){ echo "is wp_error"; }

Situation :

1. the echo is never showed.
2. wordpress crash.

Goals :

1. How do I prevent that like a misspelling error crash wordpress?
2. Why the message it’s not showed ?
3. How do I must code this specific situation to see my custom error message?