If Package Manager validation fails, fall back to "View Commands"

Problem/Motivation

This is spun off from 🐛 Package Manager validation is only necessary if the project to install is not already in the filesystem Active .

In that issue, we changed Project Browser so that Package Manager validation, which is very slow, only happens on demand -- that is, at the time you try to install a project from the UI. If there's a validation error, you get the messages in a nice modal. So far, so good.

The problem is, if there is an error, you can still press the project's "Install" button again, even though it is unlikely to succeed a second time. That's not necessarily great for usability.

Proposed resolution

This needs a little bit of thinking through from a usability perspective. If someone tries to install a project but it fails Package Manager validation, what should we do? A few options (we could do some or all of these, entirely or in combination):

• We could do nothing, and hope that subsequent attempts work (this is the status quo).
• We could dynamically change all visible projects to the fallback "View Commands" mode, by setting some global flag in the Svelte app's internal state, which is cleared if you do a full page refresh.
• We could massage the error modal to give the user information and choices about what they should do.

Let's figure this out.