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>
Merge pull request #138 from kaplun/webstyle-wsgi-capture-exceptions

WebStyle: no crash in invenio.wsgi initialisation