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>
`register_exception()` actually depends on DB being alive :-) and possibility to write to FS and/or sending email. So there are several ways in which it can go wrong. Anyway, it will go wrong immediately, on the first request (rather than on the first Apache load) :-)