Added some methods in customers controller, still working on addDiscountCardToCustomer.

src/main/java/de/rwth/swc/sqa/api/CustomerController.java

src/main/java/de/rwth/swc/sqa/api/CustomerService.java

.../rwth/swc/sqa/exceptions/ApplicationExceptionHandler.java

...de/rwth/swc/sqa/exceptions/CustomerNotFoundException.java

src/main/java/de/rwth/swc/sqa/exceptions/Error.java

...in/java/de/rwth/swc/sqa/exceptions/ExceptionResponse.java

...ava/de/rwth/swc/sqa/exceptions/InvalidInputException.java

src/test/java/de/rwth/swc/sqa/CustomerIntegrationTest.java