Errors during init get lost in the logs

[matrixbot]

This issue has been migrated from #4641.

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When started with -D, Synapse daemonizes itself before it binds any TCP/SSL listeners, which means that any errors that occur during that process (such as permission errors or "Address already in use", but also any of a number of potential configuration errors) get lost in the logs rather than being visible in stderr, and the process manager (synctl or systemd) can't tell that there was an error.