Merge pull request #6009 from mkirsano/use_p8_emission_veto

Use p8 emission veto code instead of the code inside Pythia8Interface
Update printout and make pythia8 interface explicitly fail when pythia initialization fails (instead of just silently filtering out the empty events)
Merge pull request #6458 from bendavid/py8backports_71x

Backport of powheg emission veto, plus explict initialization check for Pythia8 (71x)