makebuildserver: remove reference to f-droid.org/jessie64.box
makebuildserver: remove reference to f-droid.org/jessie64.box

Also improve error message when the initial vagrant up fails, this is
most likely due to the missing jessie64.box file.

fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroid-server