WebStyle: no crash in invenio.wsgi initialisation

* Avoids crashing in invenio.wsgi during pre-loading of data
  structures. In this way, even in case of bugs in one of the
  calls, the site can still load.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
We do. But: if `register_exception()` is broken at this level, then chances are that everything is broken, and we would notice quickly. Additionally, if it is not so terrible, than anyway this is local to the `invenio.wsgi` file so all the other `register_exception()` calls would be OK (or failing) accordingly.

The thing is we don't want the site to be completely down, just because e.g. sending email is currently broken...