Add extra data to exceptions

When Excon get's an unexpected return value, can we add the ability to put the response in the error message. Currently it looks like this:

```
Excon::Error::UnprocessableEntity: Expected([200, 201, 202, 204, 206, 304]) <=> Actual(422 Unprocessable Entity)

/Users/schneems/.gem/ruby/2.4.1/gems/excon-0.55.0/lib/excon/middlewares/expects.rb:7:in `response_call'
/Users/schneems/.gem/ruby/2.4.1/gems/excon-0.55.0/lib/excon/middlewares/response_parser.rb:9:in `response_call'
/Users/schneems/.gem/ruby/2.4.1/gems/excon-0.55.0/lib/excon/connection.rb:388:in `response'
/Users/schneems/.gem/ruby/2.4.1/gems/excon-0.55.0/lib/excon/connection.rb:252:in `request'
/Users/schneems/.gem/ruby/2.4.1/gems/heroics-0.0.22/lib/heroics/link.rb:111:in `request_with_cache'
/Users/schneems/.gem/ruby/2.4.1/gems/heroics-0.0.22/lib/heroics/link.rb:66:in `run'
/Users/schneems/.gem/ruby/2.4.1/gems/heroics-0.0.22/lib/heroics/resource.rb:28:in `method_missing'
/Users/schneems/.gem/ruby/2.4.1/gems/platform-api-2.0.0/lib/platform-api/client.rb:1231:in `update'
/Users/schneems/Documents/projects/hatchet/lib/hatchet/app.rb:50:in `block in set_config'
```

It's impossible to know if it's due to throttling, or a bad json format on my side without drinking from the EXCON_DEBUG firehose.

In this case my issue was

```
:body => "{\"id\":\"invalid_params\",\"message\":\"Cannot destroy last attachment to billing app for resource postgresql-silhouetted-85198\"}"
```

But you have to know a lot about the internals of `platform-api` to get that.