net/mail: improve detection of charset errors

net/mail: improve detection of charset errors

The detection of the "unknown charset" case was too tailored
to one specific address parser. Make it generalize, so that custom
address parsers behave the same way as the default one
for character sets they do not handle.

Fixes #41625.

Change-Id: I347d4bb6844d0a1f23e908b776d21e8be5af3874
Reviewed-on: https://go-review.googlesource.com/c/go/+/283632
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>