layman/db: Ignore an initially-missing installed file

Avoid:

  $ emerge -av app-portage/layman
  $ echo 'source /var/lib/layman/make.conf' >> /etc/portage/make.conf
  $ layman --fetch
  $ layman --list-local

   * Warning: an installed db file was not found at: ['/var/lib/layman/installed.xml']

when that's the expected behavior before you've added any local
repositories.  Instead, interpret the lack of a local file listing
installed repositories as "no repositories installed" and continue
silently on.
Merge pull request #26 from wking/allow-missing-local-installed-file

layman/db: Ignore an initially-missing installed file