PDE-1982: fix(core): ResponseError shouldn't assume response.content is always present

[eliangcs]

z.request({raw: true}), usually used for file dehydration, is failing to raise a proper ResponseError when it gets an error response. This is because the ResponseError class assumes response.content is always available, which is not true. In the prepareResponse middleware, we make reponse.content throw an error.

The fix is to be more defensive in ResponseError. When response.content throws an error, we simply keep ResponseError.content null.

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fixes #315