Map the error returned when a dial is aborted from the context package
error to the internal net package error. For example, context.Canceled
errors map to errCanceled, and context.DeadlineExceeded errors map to
poll.ErrTimeout.

Fixes #23648

Change-Id: Idf9d3d08052d540740c0b054503aaed931dc5b1e
Reviewed-on: https://go-review.googlesource.com/103518
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>