When parsing an error body allow for non-parsing error bodies

[ryber]

Describe the bug The error mapper function expects the only possibly source of a body to parse would be in the parsing exception. This is not the case if the body actually parsed correctly or the Object Mapper was extra laissez faire

To Reproduce

   @Test
    public void parsingAnAlternativeErrorObject_StringBody() {
        ErrorThing e = Unirest.get(MockServer.ERROR_RESPONSE)
                .asString()
                .mapError(ErrorThing.class);

        assertErrorThing(e);
    }