Modified handleFailureMessage to handle string-only response.

In cases where the server's response is, for instance "Unauthorized", this method currently ignores the message as it is not a JSONObject or a JSONArray. A string /is/ a valid JSON value though, legitimately returned by JSONTokener (so no exceptions are thrown and we therefore get no feedback on what went wrong). I have therefore added an extra else-if branch to call onFailure(Throwable, String) in the parent class for such situations.