BibRank: (Overflow|ZeroDivision)Error usability

* Improves handling of OverlowError, ZeroDivisionError in the bibrank
  word indexer by suggesting the administrator to schedule a regular
  reindexing of the word similarity tables.  (closes #105) (PR #2146)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>