Supertest should report response body on error

Since #95 got closed 6 years ago and there seems to be no incentive to reopen this, here's an almost 1:1 copy of that issue, but instead of reporting the hard coded `.error` property, imo the entire response body should be printed out, as currently the server response is disregarded completely.

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> At the moment when the status message does not match the expectations we get an error message like:
`expected 200 "OK, got 500 Internal server error`
It would be helpful to display a server custom error message if any, as defined by:
`res.send(500, { error: 'something blew up' });`
would display:
`expected 200 "OK", got 500 with response "{ error: 'something blew up' }"`

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> +1, but maybe we should just add the contents of res.body. It should contain the relevant error messages that would help devs figure out what the errors are.