BibRank: (Overflow|ZeroDivision)Error usability

* Improves handling of OverlowError ZeroDivisionError in the bibrank
  word indexer by properly suggest the administrator to schedule
  a regular reindexing of the word similarity tables.
  (closes #105)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>