Notifications: constrains for notif objects added

- no two TemplateMessages with the same templateId and locale can exist
- no two Regexes with the same regular expression can exist
- no two Receivers with the same target and locale can exist
- created new PerunExceptions for each case in Notification module
Notifications: skipping null values in messages

- new TemplateExceptionHandler created for skipping undefined values
  retrieving from java code
Merge pull request #563 from jirmauritz/notification

Notifications: constrains for notif objects added and skipping null values of TemplateMessage