startstopapps: Show the error message we get back from gnome-shell

... when an Eval fails
builddisks: If doing a local build, remove old data

This is actually not what we want...we should be reusing the cached
disk from last time.  Doing so just uglifies the code more.

Hm, really we should move the previous disks into cache/raw.
guestfish: Print real path to file
builddisks: Special case "local" build

Let's put things in images/local.  At the moment if you want to
use these from smoketest you have to: ln -sf local current
manifest: Update WebKitGtk

For epiphany.