console: surface `WatchDetails` errors in the UI

[hawkw]

A follow-up that I didn't address in this branch, but that we probably ought to add, is something that's displayed in the UI when we can't get details for a task. Currently, when a watch_details RPC returns an error, we log it...but logs from the console aren't visible in the UI and can only be viewed if the user redirects them to a file. So, we probably want to add something in the UI showing that there was an error.

Originally posted by @hawkw in #251 (comment)

[hawkw]

We may want to consider a fully-featured system for displaying non-fatal console internal errors in the UI. Perhaps this could piggyback on the warnings system? It would be nice to use a different icon/symbol to differentiate internal console errors from warnings about the application...