feat: add ERROR_REASON_DATASTORE_NOT_MIGRATED error code

Author: ivanauth

authzed/api/v1/error_reason.proto

@@ -413,4 +413,17 @@ enum ErrorReason {
   //       }
   //     }
   ERROR_REASON_TRANSACTION_METADATA_TOO_LARGE = 29;
+
+  // The request failed because the datastore has not been migrated.
+  // This typically indicates that the `spicedb datastore migrate` command
+  // has not been run against the backing database.
+  //
+  // Example of an ErrorInfo:
+  //
+  //     {
+  //       "reason": "ERROR_REASON_DATASTORE_NOT_MIGRATED",
+  //       "domain": "authzed.com",
+  //       "metadata": {}
+  //     }
+  ERROR_REASON_DATASTORE_NOT_MIGRATED = 30;
 }