If I put a 10 millisecond sleep at testHookWaitResLoop, before the big
select in (*persistConn).roundTrip, two flakes immediately started
happening, TestTransportBodyReadError (#19231) and
TestTransportPersistConnReadLoopEOF.

The problem was that there are many ways for a RoundTrip call to fail
(errors reading from Request.Body while writing the response, errors
writing the response, errors reading the response due to server
closes, errors due to servers sending malformed responses,
cancelations, timeouts, etc.), and many of those failures then tear
down the TCP connection, causing more failures, since there are always
at least three goroutines involved (reading, writing, RoundTripping).

Because the errors were communicated over buffered channels to a giant
select, the error returned to the caller was a function of which
random select case was called, which was why a 10ms delay before the
select brought out so many bugs. (several fixed in my previous CLs the past
few days).

Instead, track the error explicitly in the transportRequest, guarded
by a mutex.

In addition, this CL now:

* differentiates between the two ways writing a request can fail: the
  io.Copy reading from the Request.Body or the io.Copy writing to the
  network. A new io.Reader type notes read errors from the
  Request.Body. The read-from-body vs write-to-network errors are now
  prioritized differently.

* unifies the two mapRoundTripErrorFromXXX methods into one
  mapRoundTripError method since their logic is now the same.

* adds a (*Request).WithT(*testing.T) method in export_test.go, usable
  by tests, to call t.Logf at points during RoundTrip. This is disabled
  behind a constant except when debugging.

* documents and deflakes TestClientRedirectContext

I've tested this CL with high -count values, with/without -race,
with/without delays before the select, etc. So far it seems robust.

Fixes #19231 (TestTransportBodyReadError flake)
Updates #14203 (source of errors unclear; they're now tracked more)
Updates #15935 (document Transport errors more; at least understood more now)

Change-Id: I3cccc3607f369724b5344763e35ad2b7ea415738
Reviewed-on: https://go-review.googlesource.com/37495
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>