Don't convert errors to JSON if they are JSON already
Don't convert errors to JSON if they are JSON already

For example, if the identity server throws a 401 on `/account`, we end up here with a JSON object. Don't convert the string `object Object` to JSON because it'll fail: just use the object.
matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk