dokan: handle std::stoul exceptions

dokan: handle std::stoul exceptions

We're using std::stoul to parse cli args, however we aren't
catching the exceptions.

This change will handle the exceptions and log the according
error message. For consistency, we'll use the std conversion
functions throughout ceph-dokan.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>