software_manager: Search dict with lowercase distro name, add logging

The generic distribution package list is stored by with a lowercase
distro name key, using the detected_distro.name value unmodified on
Ubununtu results in failure as Ubuntu != ubuntu. Add some basic
logging to make this sort of failure much easier to identify in the
future.

V2: Use preferred logging syntax (arg list instead of % operator)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>