Add not implemented response

Author: courtneyeh

Diff for: validator/client/src/main/java/tech/pegasys/teku/validator/client/restapi/apis/DeleteGraffiti.java

@@ -40,6 +40,7 @@ public DeleteGraffiti() {
                 "Successfully removed the graffiti, or there was no graffiti set for the requested public key.")
             .withAuthenticationResponses()
             .withNotFoundResponse()
+            .withNotImplementedResponse()
             .build());
   }
 

Diff for: validator/client/src/test/resources/tech/pegasys/teku/validator/client/restapi/paths/_eth_v1_validator_{pubkey}_graffiti.json

@@ -52,6 +52,16 @@
           }
         }
       },
+      "501" : {
+        "description" : "Not implemented",
+        "content" : {
+          "application/json" : {
+            "schema" : {
+              "$ref" : "#/components/schemas/HttpErrorResponse"
+            }
+          }
+        }
+      },
       "400" : {
         "description" : "The request could not be processed, check the response for more information.",
         "content" : {