Fail if there is an error retrieving the user + extra test (#142)

Previously this would fail, but permit the request, which isn't
normally what you'd want.

internal/server.go

@@ -167,6 +167,7 @@ func (s *Server) AuthCallbackHandler() http.HandlerFunc {
 		user, err := p.GetUser(token)
 		if err != nil {
 			logger.WithField("error", err).Error("Error getting user")
+			http.Error(w, "Service unavailable", 503)
 			return
 		}