More detailed errors when connection closed during message body

[njsmith]

Intended as at least a first step towards gh-23.

[njsmith]

@Lukasa: so here's a first step towards fixing #23, with "human readable" errors only -- I guess we can merge this first and then decide whether to add special exception types as well.

The exception message phrasing currently a bit awkward IMO -- suggestions welcome.

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #25 into master will not change coverage

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[njsmith]

Oh, and to be clear, the full behavior with this PR is:

• connection dropped during content-length body: "peer closed connection without sending complete message body (received N bytes, expected M)"

• connection dropped during a chunked body while we are waiting for the \r\n at the end of a chunk header, or while reading trailers: "peer unexpectedly closed connection" (generic fallback)

• connection dropped during chunked body, at any other moment: "peer closed connection without sending complete message body (incompleted chunked read)"

[Lukasa]

So for my part I think this would be a good first step. I can investigate with urllib3 to see if this gets me close enough to what we already had to satisfy my needs. =D

[njsmith]

Cool, let's do this then. (As usual, let me know if you need me to roll a release.)