As far as I can tell, setting 'suppressExceptions' to true will get overridden to false if devMode is true.
This line in init()...
  
  
    
        
          |  | $this->suppressExceptions = !\Craft::$app->getConfig()->general->devMode; | 
    
   
 
... will overwrite 
suppressExceptions to false if 
devMode is true, regardless of what you specify for 
suppressExceptions explicitly.