Additional "invalid credential" message displayed
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Issue body
When I try to run a store command without having run snapcraft login first, now the message printed is this:
$ snapcraft list-keys
No valid credentials found. Have you run "snapcraft login"?
Invalid credentials: Root macaroon not in the config file.
The second message is useless, and talks about a macaroon that's a detail a user shouldn't need to know.
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