message: [lang] Slightly improve error message for missing deploy handler
diff: @@ -92,7 +92,7 @@ impl Contract {
  return Err(
  SynError::new(
  Span::call_site(),
  - "could not find deploy handler (`on_deploy` message)"
  + "could not find contract deploy handler: `on_deploy`"
  ).into()
  )
  }
  