nettest: log more details about err for checkForTimeoutError
nettest: log more details about err for checkForTimeoutError

For golang/go#52893

Change-Id: I2897c3f22b670af0aa2df1edbc9bbebdf1bafe59
GitHub-Last-Rev: bdccfc34beedc504a7ab4b3e903f1e09753ff85f
GitHub-Pull-Request: golang/net#132
Reviewed-on: https://go-review.googlesource.com/c/net/+/406435
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Auto-Submit: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/net