Add docs link to LXD server errors

I don't think we should include links directly in error messages from the backend.

These errors can also appear from API calls, which means the caller would get an HTML snippet in the text.
Also note that we're not always in control of the error message we display, in the specific case the error might come from LXD, and MAAS just pushes it up.

Couldn't the UI just add the link text when an error is displayed from the connect form?

I think the UI has the same issue of not know what the error is related to, but maybe it's fine to have the link for any error. Caleb, any thoughts about that?

Alberto, we have links come from the API for other errors/notifications. Do we need to think about changing them as well if they're resulting in html appearing in the console?

I agree it's an odd pattern sending an HTML snippet from the API. We should probably change any errors and notifications that currently do the same thing.

For now I think it's fine to just show the same link for any error that happens when connecting to a LXD server fails. We'll have to special case the Add KVM form for now, but maybe in the future it would be good to be able to associate a link with any action.