Make lintian task fail_on_severity: error by default, instead of none

The current lintian task available in debusine.d.n (0.11.1.dev11+g8cd5e6bb) defaults to fail_on_severity: none, but as maintainer, I would prefer that it fails on error.

I find it inaccurate that a lintian tasks succeeds when there are Error tags, e.g. in: https://debusine.debian.net/debian/developers/work-request/85603/.

Actually, I would find it helpful if there was a "lintian regression" task, that compares the outcome of lintian with the status of a previous package revision (using the same lintian version). So I could easily know if I am introducing new errors, warnings, ... etc. But that is another issue.