Add impls for Error trait for Rust reqwest

[aidanhs]

Adds Error trait impls for the common error types so that they can be chained via normal rust error handling libraries.

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cc @frol (2017/07) @farcaller (2017/08) @richardwhiuk (2019/07) @paladinzh (2020/05)

[wing328]

@aidanhs thanks for the PR. Let's give it a try.