Flush error logs before process exits

Issue Details

Config errors are not written to stdout (or similar) before the process exits. The issue is simply that the error is logged but not flushed so the information is never written.

The proper fix would probably be to just add a flush after this line: https://github.com/caddyserver/caddy/blob/master/cmd/cobra.go#L156

Or, move the error logging to before the flushing that happens: https://github.com/caddyserver/caddy/blob/master/cmd/commandfuncs.go#L254

Since this happens in more than one place the proper way would probably be to just ensure all buffered messages are flush prior to the process terminating.

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