Fix generation of list of package files to exclude in the standard prepare script

c714a331 introduced an annoying bug here. If the filenames piped to xargs include
an apostrophe, xargs will complain and stop. We have thus, in practice, been
including all files that appear in the list after

/usr/share/sounds/ubuntu/ringtones/Sam's Song.ogg

!! Also, we've not been properly handling filenames with spaces in, and perhaps
other filenames too. We thus null-terminate the filenames for ourself.

Because I want to get to the point where we can have a sensible return code
from the prepare script, this commit also adds some set -e commands to try
to ensure errors bubble up. This is a little tedious to achieve due to all
the subshells in this script.

For the same reason, we split off the "diff" command into a "set +e" block
because diff returns non-zero if differences are found. We do not consider
that to be an error!