Expose HTTP response to end users

[Acconut]

Is your feature request related to a problem? Please describe.

Users currently cannot retrieve the response body. This includes successful responses (e.g. after an upload is finished), but also error cases, where the error message only includes the status code. The response body is not included in the message, although it usually includes more details.

Describe the solution you'd like

• Error messages from 4xx status codes should include the response body
• TusClient should have a callback that can be override to handle the response object in a custom manner.

See #89, #59, #80, for example.