extensions-app: Improve "noshell" status page

When running Extensions outside GNOME, a status page appears informing that the extensions can't be listed. However, the status page has some UI issues:

• It doesn't have an icon
• Doesn't use header capitalization
• Using "we're very sorry" in the description is an uncommon phrasing tone that isn't used in other GNOME apps. The description is also too verbose
• "Something's gone wrong" is a vague explanation

Fix all of the issues above.

Before	After