errorlib: context support in sentry

* Introduces support for HTTP, and user context in sentry, by
  exploiting information provided in req object when available.
  (closes #1960) (PR #2147)

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