Add "sync" to file not found error message

[stakats]

When an attachment isn't found, the dialog should also note that the file may not have yet been synced, e.g.

"The attached file could not be found. It may not yet be synced to this device, or it may have been moved or deleted outside of Zotero."

[ajlyon]

The corresponding message for items that have not been synced and which are referenced in word-processed documents should be changed in the same way, as Mark notes: ​http://forums.zotero.org/discussion/17660/