[#190] status: add Object.OBJECT_NOT_FOUND code

`OBJECT_` prefix is needed because of possible conflicts with other
`NOT_FOUND` codes declared in the same file.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

status/types.proto

@@ -97,4 +97,7 @@ enum Object {
     // Details:
     // - [**0**] Human-readable description.
     ACCESS_DENIED = 0;
+    // [**2049**] Object not found.
+    OBJECT_NOT_FOUND = 1;
+}
 }