Enhance Slack webhook message readability

[TeemuSuoranta]

Is your feature request related to a problem? Please describe.
You can include a Slack webhook URL that is pinged when updates fail. This is a much appreciated feature but at the moment reading these notification can be difficult to read as there is no repeating format and the site name is not emphasised.

Describe the solution you'd like
Create a format for these messages that makes it easy to look through these messages that might be pointed to a single Slack channel.

For example:

**{Site name}** {Site URL}: Cannot proceed with updating as the live site has errors that need to be addressed first

Seravo https://seravo.fi: Cannot proceed with updating as the live site has errors that need to be addressed first

If it is possible to point to additional info, it could be also like:

**{Site name}** {Site URL}: Cannot proceed with updating as the live site has errors that need to be addressed first. See more info at {URL to admin page}

The actual sending logic is outside of this plugin and probably inside some server scripts so I couldn't make a PR for these changes.

[TeemuSuoranta]

As there are notifications for both successful and failed attempts, you could also prefix the messages by emojis like 🆘 / ✅ so that issues can be easier to spot from other messages.

[ottok]

Thanks, good ideas!