Fix issue with ResponseError not exposing it's `message` method publicly

This was causing a further exception when raising the `ResponseError` exception saying

```
rspec-core-2.13.1/lib/rspec/core/formatters/base_text_formatter.rb:264:in `dump_failure_info': 
private method `message' called for #<Bcx::ResponseError:0x007fcadbe08960> (NoMethodError)

``` 

Changed to use the standard `to_s` which is automatically called when raising.