Invalid API endpoint exception improvement

MAILPOET-6022

mailpoet/lib/API/JSON/API.php

@@ -269,10 +269,17 @@ class API {
   }
 
   public function createErrorResponse($errorType, $errorMessage, $responseStatus) {
+    $errorMessages = [
+      $errorType => $errorMessage,
+    ];
+
+    if ($errorType === Error::BAD_REQUEST) {
+      $mpReinstallErrorMessage = __('The plugin has encountered an unexpected error. Please reload the page. If that does not help, re-install the MailPoet Plugin. See: https://kb.mailpoet.com/article/258-re-installing-updating-the-plugin-via-ftp', 'mailpoet');
+      $errorMessages[Error::REINSTALL_PLUGIN] = $mpReinstallErrorMessage; // not all places on the frontend accept html for an error message. we want this to be backwards compatible
+    }
+
     $errorResponse = new ErrorResponse(
-      [
-        $errorType => $errorMessage,
-      ],
+      $errorMessages,
       [],
       $responseStatus
     );