More visible warnings in output

### What problem does your feature request solve?

Provide a way for recipes to output user visible warnings that can be seen on the output webpage.

### Describe the solution you'd like

I suspect this will hook into the mechanism proposed for #558. Maybe it should also have additional styling to highlight it.

### Describe alternatives you've considered

We currently just log it to the workflow logs, which isn't very discoverable. I don't think cylc has a notification mechanism, but we could send an email.

Start by reviewing the mechanism proposed in issue #558, then trace how recipes currently write to workflow logs and how those results appear on the output webpage. Define the warning behavior, visibility and styling requirements, and the conditions that would demonstrate the feature is complete.

Written by the indexing model from the issue text.