log/syslog: report hostname mismatch error details

log/syslog: report hostname mismatch error details

The existing error log in check doesn't report the got/want hostname
even though that can be the cause of the error. Log those as well.

While we're here, also report os.Hostname() errors.

For #59568.

Change-Id: Ia277f85eddc541f2e78d719bc731db24e4513754
Reviewed-on: https://go-review.googlesource.com/c/go/+/483915
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>