fileIsShared fails on sites with shared file repo

"missing" exists in page dict when the file is shared.

KeyError occurs for file on a shared repo if the file
is not a local file.  To prevent KeyError, check if
'imageinfo' exists first, then if 'missing' exists
the page does not exist.

Bug: 70494
Change-Id: I2cfbbd2434cb03600c9d53e648e0534f05756d46
Merge "fileIsShared fails on sites with shared file repo"