errorlib: fix Sentry context syntax issue

* Fixes Python syntax issue that slipped in the Sentry context support
  commit d73bd94b2fbe8666af31dfec25c334c466e6f6df.
  (addresses #1960) (PR #2147)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
BibField: CFG_BIBFORMAT_HIDDEN_RECJSON_FIELDS

* Introduces ad-interim new config variable
  CFG_BIBFORMAT_HIDDEN_RECJSON_FIELDS to list which recjson fields
  need to be not exposed via public interfaces.
  (addresses #2197)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>