Shutdown with causing error.

[hsblhsn]

The idea is mostly from #672 and my comment on that issue.

We can provide a way to log the error on exit with fxlog if any error provided with ShutdownOption.

What do you think?

[abhinav]

This sounds like a good idea.
We're already considering a ShutdownCode(int) option in #912,
so a ShutdownCause(error) that is then then returned by app.Err() makes sense.