message: Warning msg starts with "WARNING" in config binding
  Summary: Make it less confusing when there's other error.
diff: #define error_or_warn(error, warn, msg, ...) \
  always_assert_log(!(error), msg, ##__VA_ARGS__); \
  if (warn) { \
  - fprintf(stderr, msg, ##__VA_ARGS__); \
  + fprintf(stderr, "WARNING: " msg, ##__VA_ARGS__); \
  }
  namespace {
  