hwclock: report rtc open() errors on --verbose

hwclock: report rtc open() errors on --verbose

Let's be more verbose and provide real open() error to make
debugging easier on --verbose.

For example:

$ hwclock --verbose
hwclock from util-linux 2.34.193-6bebea-dirty
System Time: 1570445823.701266
Trying to open: /dev/rtc0
hwclock: cannot open /dev/rtc0: Permission denied          <---
No usable clock interface found.
hwclock: Cannot access the Hardware Clock via any known method.

Addresses: https://github.com/karelzak/util-linux/issues/879
Signed-off-by: Karel Zak <kzak@redhat.com>