Fix instructions on how to setup caching

The rest of the script expects $HOME to be expanded,
so expand it as part of the shell run.
Print error hint when add_changes_entry() fails

Previously add_changes_entry() simply returned 0 when
it failed without additional details. Change it to the
die/eval model so that an error hint can also be passed
around. This has also the advantage that an error case
can not be accidentally ignored.