PM / Hibernate: Make some boot messages look less scary

The hibernate resume code checks if there is an image to resume from
on every boot and, if the kernel is built with CONFIG_PM_DEBUG set
and the image is not present, it prints some scary messages
suggesting there was a boot error of some sort.  Apparently, some
users are confused by them, so make them look less scary and adjust
the other hibernate resume debug messages to match them.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>