Ensure workflow was trashed before deleting it

[MAILPOET-4540]

mailpoet/lib/Automation/Engine/Exceptions.php

@@ -29,6 +29,7 @@ class Exceptions {
   private const WORKFLOW_STEP_MODIFIED_WHEN_UNKNOWN = 'mailpoet_automation_workflow_step_modified_when_unknown';
   private const WORKFLOW_NOT_VALID = 'mailpoet_automation_workflow_not_valid';
   private const MISSING_REQUIRED_SUBJECTS = 'mailpoet_automation_missing_required_subjects';
+  private const WORKFLOW_NOT_TRASHED = 'mailpoet_automation_workflow_not_trashed';
 
   public function __construct() {
     throw new InvalidStateException(
@@ -204,4 +205,11 @@ class Exceptions {
         )
       );
   }
+
+  public static function workflowNotTrashed(int $id): UnexpectedValueException {
+    return UnexpectedValueException::create()
+      ->withErrorCode(self::WORKFLOW_NOT_TRASHED)
+      // translators: %d is the ID of the workflow.
+      ->withMessage(sprintf(__("Can't delete workflow with ID '%d' because it was not trashed.", 'mailpoet'), $id));
+  }
 }