json/read-str can throw exception on invalid JSON content

[iarenaza]

do-request uses json/read-str to convert the response of the request into a Clojure data structure and return it as the :body value (https://github.com/magnetcoop/dashboard-manager.grafana/blob/0acf4f238e8dbd6e26f54208639830cdcfb625bf/src/magnet/dashboard_manager/grafana.clj#L70).

But if we get invalid JSON from the remote endpoint, it throws an exception. And we don't catch it and produce a suitable return value for the method.