Compute _has_errors only from _error_areas
Compute _has_errors only from _error_areas

This way we need fewer intermediary variables. It's also more robust and somewhat more elegant.

Contributes to issue CURA-2625.

hmflash/Cura,ynotstartups/Wanhao,Curahelper/Cura,fieldOfView/Cura,Curahelper/Cura,ynotstartups/Wanhao,fieldOfView/Cura,hmflash/Cura