PM: sleep: Refine error message in try_to_freeze_tasks()

A previous change amended try_to_freeze_tasks() with the "what"
variable pointing to a string describing the group of tasks subject to
the freezing which may be used in the error message in there too, so
make that happen.

Accordingly, update sleepgraph.py to catch the modified error message
as appropriate.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>