errorlib:  support for Sentry logging

* Optionally captures exceptions and sends them to the Sentry logging
  platform, if directed by the new CFG_ERRORLIB_SENTRY_URI configuration
  variable. (closes #1726)

* Adds raven python package to requirements-extra

Signed-off-by: Fredrik Nygård Carlsen <me@frecar.no>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>