Log the original bind exception when encountering Failed to listen on 0.0.0.0, continuing because listening on [::]

[MadLittleMods]

Log the original bind exception when encountering Failed to listen on 0.0.0.0, continuing because listening on [::]

Before:

WARNING - call_when_running - Failed to listen on 0.0.0.0, continuing because listening on [::]

After:

WARNING - call_when_running - Failed to listen on 0.0.0.0, continuing because listening on [::]. Original exception: CannotListenError: Couldn't listen on 0.0.0.0:8008: [Errno 98] Address already in use.

Spawning from @gaelgatelement encountering this error and seeing that :: is not actually binding to both IPv4 and IPv6 as this comment suggests. While this PR doesn't change or fix that, this just gives us more insight and context when we see the warning:

synapse/synapse/app/__init__.py

Lines 46 to 48 in 3aaa2e8

	Binding on both 0.0.0.0 and :: causes an exception on Linux and macOS
	because :: binds on both IPv4 and IPv6 (as per RFC 3493).
	When binding on 0.0.0.0 after :: this can safely be ignored.

Testing strategy

1. Edit your homeserver config (homeserver.yaml) to bind both :: and 0.0.0.0

   listeners:
     - port: 8008
       bind_addresses: [
         '::',
         '0.0.0.0'
       ]
       # ...

2. Start Synapse: poetry run synapse_homeserver --config-path homeserver.yaml
3. Notice the warning in your homeserver logs (homeserver.log), WARNING - call_when_running - Failed to listen on 0.0.0.0, continuing because listening on [::]

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[MadLittleMods]

Thanks for the review @devonh 🐝