Clear error when rerunning migration

[MAILPOET-4811]

mailpoet/lib/Migrator/Store.php

@@ -29,7 +29,9 @@ class Store {
       VALUES (?, current_timestamp())
       ON DUPLICATE KEY UPDATE
         started_at = current_timestamp(),
-        retries = retries + 1
+        completed_at = NULL,
+        retries = retries + 1,
+        error = NULL
     ", [$name]);
   }