show nicer message for unauthed users who try to use extensions

Created by: sqs

Depends on https://github.com/sourcegraph/extensions-client-common/pull/65.

Previously, users would see a terse message like "to toggle coverage or edit settings, you must sign in". This was not very helpful, especially when the user was on GitHub (sign in where? GitHub? I am signed in!).

Now it reads as follows:

Unable to update user setting git.blame.lineDecorations because you are not signed in.

Sign into Sourcegraph on localhost:3080

I added the setting key to make it sound less like spam and more like a real necessity (which it is).