Failing to deliver a message should fail gracefully

[chuck-sys]

Summary

Failing to deliver a message should fail gracefully, with a proper error message and handler, instead of without one, which caused a production deployment issue.

Reproduction steps

1. Follow dev guide
2. Change default channels to newly created channels that Rocket has yet seen (or been invited to)
3. Start Rocket

Expected behaviour

Logs that clearly say what is wrong (in this case, that we couldn't send the Rocket has restarted successfully :clap: :clap:). In this case to correct it, we just install the app into the channel.

Context

We also will want to update the documentation to reflect this. Add that the developer will have to install the app into all the specified channels in order to see anything.