Submit a test pull query on Mapping edit form validate to make sure the where clause works ok.

modules/salesforce_mapping/src/Form/SalesforceMappingFormCrudBase.php

@@ -6,6 +6,8 @@ use Drupal\Core\Ajax\AjaxResponse;
 use Drupal\Core\Ajax\ReplaceCommand;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\salesforce_mapping\MappingConstants;
+use Drupal\salesforce\Event\SalesforceEvents;
+use Drupal\salesforce\Event\SalesforceErrorEvent;
 
 /**
  * Salesforce Mapping Form base.
@@ -258,6 +260,16 @@ abstract class SalesforceMappingFormCrudBase extends SalesforceMappingFormBase {
       // @TODO replace with Dependency Injection
       $form_state->setError($element, $this->t('%name field is required.', ['%name' => $element['#title']]));
     }
+    // dpm($form_state->getValues());
+    if ($this->entity->doesPull()) {
+      try {
+        $testQuery = $this->client->query($this->entity->getPullQuery());
+      }
+      catch (\Exception $e) {
+        $form_state->setError($form['pull']['pull_where_clause'], $this->t('Test pull query returned an error. Please check logs for error details.'));
+        \Drupal::service('event_dispatcher')->dispatch(SalesforceEvents::ERROR, new SalesforceErrorEvent($e));
+      }
+    }
   }
 
   /**

src/Rest/RestException.php

@@ -16,8 +16,21 @@ class RestException extends \RuntimeException implements ExceptionInterface {
    *
    */
   public function __construct(ResponseInterface $response, $message = "", $code = 0, Throwable $previous = NULL) {
-    parent::__construct($message, $code, $previous);
     $this->response = $response;
+    $message .= $this->getResponseBody();
+    parent::__construct($message, $code, $previous);
+  }
+
+  public function getResponse() {
+    return $this->response;
+  }
+
+  public function getResponseBody() {
+    $body = $this->response->getBody();
+    if ($body) {
+      return $body->getContents();
+    }
+    return '';
   }
 
 }