modified edit

- changed edit so that it will terminate if either delete or add fail
- it will also throw different exceptions depending on which exception
was thrown from add or delete
- else it will throw a default exception
added constants

- added constants for edit
- added constants for exception handling
changed strings to constants
refactored add

- moved condition for checking for free slots into a method called
querySlots(Task task)
renamed querySlots

- renamed to isFreeTimeslots as it is a boolean method
refactored delete

- moved condition for checking if task exists into isExistingTask(Task
task)
Merge remote-tracking branch 'origin/logic'