Fix problem with non-existing linked files folder (FWNX-1005)

Another attempt to fix the problem when the linked files folder
doesn't exist. This can happen when the project gets moved between
machines. The previous attempt caused a dialog box to pop up
recursively causing it not to show anything. We now check for the
existence of the linked files folder when we open the cache.

Cherry-picked from 919a53 on develop.

Change-Id: I9d2a91dcb35c57433ea574a34520f270560811db