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>