functional-tests: Fix 'not a directory' error in writeback tests

This makes the following error less likely, and clearer when it does
occur:

    (tracker-miner-fs:28886): Tracker-WARNING **: Could not open directory 'file:///home/sam/tracker-tests/writeback': Not a directory
writeback: Improve error when writeback fails due to missing module
functional-tests: Remove another guarantee-metadata dependency
Merge branch 'sam/functional-test-50x-fixes'