crypto/tls: wrap KeyLogWriter errors w/ prefix

If a tls.Config populates a KeyLogWriter, and the io.Writer's Write()
returns an error when using it in writeKeyLog() we should wrap the error
with a prefix that indicates the source to aid in debugging.

Fixes #79392

Change-Id: Ie16bb2908575123a8e80c623a15f3835a7bc3e92
Reviewed-on: https://go-review.googlesource.com/c/go/+/784740
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Neal Patel <neal@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>