errorlib: wrap warnings to invenio.err

* Redirects captured warnings and corresponding
  tracebacks directly into invenio.err rather
  than polluting stderr and consequently apache.err.
  (closes #1616)

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