Make sure errors from `Promise.sync()` and/or `PromisingFunction.run()` are informative enough so the user knows when to use which

[teremterem]

• When .sync() is used outside of a running event loop, the error message should advise to use .run() instead.
• What to do if it's the other way around (.run() is used, but the the event loop is already running) ?
  • Should await_children()/await_children_sync() be also mentioned in any way ?
• Generate tests for these scenarios.