database/sql: fix regression from earlier RawBytes fix

database/sql: fix regression from earlier RawBytes fix

The earlier CL 497675 for #60304 introduced a behavior change
that, while not strictly a bug, caused a bunch of test failures
in a large codebase. Rather than add behavior changes in a 10 year
old package, revert to the old behavior: a context cancelation
between Rows.Next reporting false and a call to Rows.Err should
not result in Rows.Err returning the context error.

That behavior was accidentally added in CL 497675 as part of changing
how contexts and Rows iteration worked.

Updates #60304
Updates #53970

Change-Id: I22f8a6a6b0b5a94b430576cf50e015efd01ec652
Reviewed-on: https://go-review.googlesource.com/c/go/+/498398
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>