tracker-extract: More helpful error when missing GStreamer plugins

If extraction with GstDiscoverer fails due to lack of a plugin, we can
display exactly what plugin was missing rather than just giving a
generic error. This makes it a bit easier to fix the problem.
functional-tests: Don't require 'guarantee-metadata' for some tests

Tests shouldn't break just because --enable-guarantee-metadata is not
specified at configure time. This feature is off by default so we
shouldn't depend on it. See GB#733536.
Merge branch 'sam/functional-test-40x-fixes'