[Rivet] Broken HepMC 2.3. and 2.4 builds

[Andy Buckley]

Hi guys,

Eike, I think you've made some change in the Rivet Run class which has 
broken the Rivet builds against HepMC 2.3 and 2.4 (I get lots of 
automated emails when this happens!):

Run.cc: In member function ‘bool Rivet::Run::readEvent()’:
Run.cc:37: error: ‘class HepMC::IO_GenEvent’ has no member named 
‘error_message’

I don't know what this change has done, but can you please make sure 
it's done in a way that is backwards compatible?

Thanks,
Andy

-- 
Dr Andy Buckley
SUPA Advanced Research Fellow
Particle Physics Experiment Group, University of Edinburgh