refactor: change default implementations of build and update

Author: metacosm

operator-framework-core/src/main/java/io/javaoperatorsdk/operator/api/reconciler/DependentResource.java

@@ -7,8 +7,11 @@
 
 public interface DependentResource<R extends HasMetadata> {
   default R build() {
-    throw new UnsupportedOperationException(
-        "Should be implemented only if DependentResource is configured as creatable");
+    if (getConfiguration().creatable()) {
+      throw new IllegalStateException(
+          "Should be implemented if DependentResource is configured as creatable");
+    }
+    throw new IllegalStateException("Should not be called if DependentResource is not creatable");
   }
 
   ResourceCache<R> getCache();
@@ -20,7 +23,11 @@ default R fetchFor(HasMetadata owner) {
   DependentResourceConfiguration<R> getConfiguration();
 
   default R update(R fetched) {
-    return fetched;
+    if (getConfiguration().updatable()) {
+      throw new IllegalStateException(
+          "Should be implemented if DependentResource is configured as updatable");
+    }
+    throw new IllegalStateException("Should not be called if DependentResource is not updatable");
   }
 
 }