Improve failure checking for Lmod cache update script

[bedroge]

If the Lmod cache update script encounters an error, it just bails out completely. We may want to continue, or otherwise revert the entire transaction (in order to keep things consistent).

Related to this, new architectures seem to get the wrong permissions for their .lmod directory. Would be nice to fix that as well. This actually happened for EESSI/docs#422, as grace was introduced.