BibCheck: improve url plugin

* Deal with exceptions from requests library

* Avoid querying CFG_SITE_URL and CFG_SITE_SECURE_URL
BibMatch: Fix validator problem

* Fixes a problem in the validator while retrieving values from
  records that could cause an empty string to be evaluated as
  a real value.

Reported-by: Raja Sripada <rsripada@cern.ch>
DocExtract: Page-end extracted from references
 * RefExtract preserves page-end, also added
   extract_journal_reference() function to get reference data
   from single strings.
RefExtract: Refactored output from extract_journal_reference