Add popup error handler abstract class, use that by default for errors w...

...e want to display, fix the non handled errors in stage stuff.

Conflicts:
	TaskManager/src/main/java/edu/wpi/cs/wpisuitetng/modules/taskmanager/views/NewStageView.java


https://trello.com/c/MuH7k9bl/163-doesn-t-display-popup-on-error-when-deleting-stages