BibFormat: do not exit if no XSLT processor found
BibFormat: do not exit if no XSLT processor found

* Do not sys.exit when there is no XSLT processor available.
  (addresses #477)

* Cherry-picked from 546aa7408a54c35202bf89f17dfa69f547c773ba.

inveniosoftware/invenio-formatter,tiborsimko/invenio-formatter,tiborsimko/invenio-formatter,inveniosoftware/invenio-formatter,inveniosoftware/invenio-formatter,tiborsimko/invenio-formatter