Only try to use CakeErrorController if AppController exists.
Only try to use CakeErrorController if AppController exists.

This fixes missing error pages when there are parse errors in
AppController.

Fixes #3850

garethellis36/cakephp,tisu2tisu/cakephp,michaelze/cakephp,cakephp-fr/cakephp,tgr0ss/cakephp,khoimt/cakephp-study,erikfig/cakephp,ndm2/cakephp,jeffblack360/cakephp,CakeDC/cakephp,ADmad/cakephp,mvdriel/cakephp,kumamidori/cakephp,jerems02/cakephp,jjj117/cakephp,akintoey/cakephp,ndm2/cakephp,Balachan27/cakephp,bravo-kernel/cakephp,akintoey/cakephp,eboominathan/cakephp,CVO-Technologies/cakephp,pbaire/cakephp,cakephp/cakephp,akintoey/cakephp,dkarvounaris/cakephp,berrygoudswaard/cakephp,Phillaf/cakephp,et304383/passbolt_api,erikfig/cakephp,CakeDC/cakephp,et304383/passbolt_api,web-player/cakephp,Phillaf/cakephp,dakota/cakephp,kumamidori/cakephp,elboletaire/cakephp,dakota/cakephp,nrother/cakephp,eboominathan/cakephp,dakota/cakephp,burriko/cakephp,pbaire/cakephp,timhoeppner/cakephp,jjj117/cakephp,berrygoudswaard/cakephp,HavokInspiration/cakephp,tisu2tisu/cakephp,eryw/cakephp,Balachan27/cakephp,ADmad/cakephp,yinhe007/cakephp,justinyost/cakephp,CVO-Technologies/cakephp,adopico/cakephp,bravo-kernel/cakephp,CakeDC/cakephp,timhoeppner/cakephp,tenkoma/cakephp,HavokInspiration/cakephp,elboletaire/cakephp,jeffblack360/cakephp,petteyg/cakephp,HavokInspiration/cakephp,cakephp-fr/cakephp,michaelze/cakephp,web-player/cakephp,cakephp/cakephp,eboominathan/cakephp,garethellis36/cakephp,tenkoma/cakephp,nrother/cakephp,jjj117/cakephp,yinhe007/cakephp,timhoeppner/cakephp,burriko/cakephp,dkarvounaris/cakephp,michaelze/cakephp,jerems02/cakephp,adopico/cakephp,mvdriel/cakephp,ADmad/cakephp,web-player/cakephp,tisu2tisu/cakephp,yinhe007/cakephp,mvdriel/cakephp,thinkingmedia/cakephp,et304383/passbolt_api,cakephp/cakephp,eryw/cakephp,tgr0ss/cakephp,ionas/cakephp,davidyell/cakephp,gmponos/cakephp,Balachan27/cakephp,ndm2/cakephp,ndm2/cakephp,nrother/cakephp,gmponos/cakephp,petteyg/cakephp,tenkoma/cakephp,dkarvounaris/cakephp,petteyg/cakephp,tenkoma/cakephp,adopico/cakephp,gmponos/cakephp,Phillaf/cakephp,ionas/cakephp,khoimt/cakephp-study,thinkingmedia/cakephp,garethellis36/cakephp,erikfig/cakephp,eryw/cakephp,cakephp/cakephp,cakephp-fr/cakephp,berrygoudswaard/cakephp,jerems02/cakephp,et304383/passbolt_api,kumamidori/cakephp,ADmad/cakephp,davidyell/cakephp,dakota/cakephp,elboletaire/cakephp,et304383/passbolt_api,justinyost/cakephp,CVO-Technologies/cakephp,petteyg/cakephp,jeffblack360/cakephp,ionas/cakephp,garethellis36/cakephp,mvdriel/cakephp,davidyell/cakephp,CakeDC/cakephp,tgr0ss/cakephp,khoimt/cakephp-study,thinkingmedia/cakephp,pbaire/cakephp,justinyost/cakephp,bravo-kernel/cakephp,burriko/cakephp