Use better description for size when size is not known

Summary:
Show that size is being fetched instead of displaying size 0 which can confuse users. Also
when we fail to fetch the size, then display "unknown size" rather then again showing
that size is 0.

Reviewers: apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5219