snapcraft export-login doesn't provide meaningful error messages when an argument is wrong

As seen in the other bug, snapcraft code basically gives a generic "Failed to get macaroon" message if the store responds with a not-OK http status (4xx and 5xx pretty much):

if response.ok:
            return response.json()["macaroon"]
        else:
            raise errors.StoreAuthenticationError("Failed to get macaroon", response)

(Hit enter too quickly, sorry).

Anyway, if snapcraft were to look at the actual payload, https://dashboard.snapcraft.io/dev/api/acl/ will typically send something detailed, such as this:

{
  "type": "devportal:v1:resource-not-found",
  "status": 404,
  "error_list": [
    {
      "message": "Snap not found for the given snap name 'shello-roadmr-1'",
      "code": "resource-not-found"
    }
  ],
  "detail": "Snap not found for the given snap name 'shello-roadmr-1'",
  "title": "Snap not found for the given snap name 'shello-roadmr-1'",
  "code": "resource-not-found"
}

An example can be seen by doing export-login with a non-existent snap name:

SNAPCRAFT_ENABLE_DEVELOPER_DEBUG=yes snapcraft export-login --channels=beta --snaps="this-is-not-a-valid-snap-name-hehehe" --expires="2019-01-01" --acls='package_push,package_release,package_access' /tmp/mac2

The exception, as we've discussed a few times in the past, are some 500 errors which aren't generated by the application server (e.g. the dreaded 503 no server available or 502 timeout) and thus aren't guaranteed to have a json error payload.