PM / core: Add error argument to dpm_show_time()

Make the core device suspend/resume code also call dpm_show_time()
on failures and add an error argument to this function so that the
message printed by it can reflect the success or failure condition.

This makes the debug messages in question look less confusing in
the failing cases.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>