Fixed #90 - better connection error handling (improve messages)

src/de/deadlocker8/budgetmaster/logic/ExceptionHandler.java

src/de/deadlocker8/budgetmaster/logic/ServerConnection.java

...locker8/budgetmaster/logic/ServerConnectionException.java

src/de/deadlocker8/budgetmaster/ui/CategoryController.java

src/de/deadlocker8/budgetmaster/ui/Controller.java

...de/deadlocker8/budgetmaster/ui/NewCategoryController.java

src/de/deadlocker8/budgetmaster/ui/NewPaymentController.java

src/de/deadlocker8/budgetmaster/ui/PaymentController.java

src/de/deadlocker8/budgetmaster/ui/SettingsController.java