Skip migrating missing profile photos

In testing, the migration was crashing when the source photo did not
exist (it is not clear where the source photo went). Instead of crashing
the import it now prints the path to the missing photo and the user info
to the console for case-by-case inspection.
Merge pull request #2 from jwalgran/topic/migrate_userphotos_skip_missing

Topic/migrate userphotos skip missing