The test was checking for 1 of 2 possible error values. But based on
goroutine scheduling and the randomness of select statement receive
cases, it was possible for a 3rd type of error to be returned.

This modifies the code (not the test) to make that third type of error
actually the second type of error, which is a nicer error message.

The test is no longer flaky. The flake was very reproducible with a
5ms sleep before the select at the end of Transport.getConn.

Thanks to Github user @jaredborner for debugging.

Fixes #16049

Change-Id: I0d2a036c9555a8d2618b07bab01f28558d2b0b2c
Reviewed-on: https://go-review.googlesource.com/24748Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>