#33 add custom exception, extended by PlantNotFoundException

src/main/java/hdm/mi/growbros/exceptions/GrowBrosException.java

+package hdm.mi.growbros.exceptions;
+
+public abstract class GrowBrosException extends RuntimeException {
+    public GrowBrosException(String message) {
+        super(message);
+    }
+}

src/main/java/hdm/mi/growbros/exceptions/PlantNotFoundException.java

 package hdm.mi.growbros.exceptions;
 
-public class PlantNotFoundException extends RuntimeException {
+public class PlantNotFoundException extends GrowBrosException {
     public PlantNotFoundException(Long id) {
         super(String.format("Plant with id '%d' was not found", id));
     }