[Rivet] Broken HepMC 2.3. and 2.4 builds

[Eike von Seggern]

On Fri, Dec 11, 2009 at 16:25 +0000, Andy Buckley wrote:
> 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?

I added this error_message print out during the debugging of the event
reading bug and thought it's not bad to keep it for future problems.
I'll revert it to the old way.

Eike