Fix ImagePage exceptions and add file tests

- Site.loadimageinfo produced KeyError on missing pages
- ImagePage.fileIsShared produced KeyError on missing pages
- Add tests to cover files, especially the different scenanios
  of local and shared file pages.

Change-Id: I7d6516aa97b71ba4b2a7a9e5f9347989ed1dd609
Merge "Fix ImagePage exceptions and add file tests"