[Rivet] debugging make-plots

[Hendrik Hoeth]

Hi guys,

at some point during fixing the normalisation issue with stacked
histograms in make-plots I got the utterly useless error message

   Plotting stacking_histograms.dat
   Error: local variable 'foo' referenced before assignment

from Python. That's it. Nothing else. I don't know why and I don't know
how we've switched off the normal Python error messages, but can I
pleeeeeaaaaaaase get them back? Knowing a line number is an incredible
for debugging. Really. And we don't need to worry about pissing off our
user base by enabling useful error messages, since they will never get
errors in the first place, right? 

Cheers,

    Hendrik

-- 
"Two equal numbers are equivalent, even if they come from two
 different people one of which you don't like"
(MLM about the theoretical top cross-section, in a meeting
about theoretical uncertainties on the top mass)