Issue #2838144 by arshadcn, Wim Leers, drpal: Update "bundle missing" error...

Issue #2838144 by arshadcn, Wim Leers, drpal: Update "bundle missing" error message in entity denormalization to indicate which field is actually missing

core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php

@@ -993,7 +993,7 @@ protected function assertNormalizationEdgeCases($method, Url $url, array $reques
       // DX: 400 when no entity type bundle is specified.
       // @todo Change to 422 in https://www.drupal.org/node/2827084.
       $response = $this->request($method, $url, $request_options);
-      $this->assertResourceErrorResponse(400, 'A string must be provided as a bundle value.', $response);
+      $this->assertResourceErrorResponse(400, sprintf('Could not determine entity type bundle: "%s" field is missing.', $bundle_field_name), $response);
     }
   }
 

core/modules/serialization/src/Normalizer/FieldableEntityNormalizerTrait.php

@@ -95,7 +95,7 @@ protected function extractBundleData(array &$data, EntityTypeInterface $entity_t
 
     // Make sure a bundle has been provided.
     if (!is_string($bundle_value)) {
-      throw new UnexpectedValueException('A string must be provided as a bundle value.');
+      throw new UnexpectedValueException(sprintf('Could not determine entity type bundle: "%s" field is missing.', $bundle_key));
     }
 
     // Make sure the submitted bundle is a valid bundle for the entity type.