fix doSM.py

Hi all,

There's a problem with doSM.py when setting up with bbb:hww-sig. It fails at the step of running the horizontal morphing for the missing H->WW signal samples in the e-mu channel. I realised that it was looking for the wrong scale_e systematic templates in some categories, which is fixed in this pull request.  It's started failing now because of the update to python/horizontal_morphing.py to fix the negative bin issue - the new function fails if the requested histogram doesn't exist.

cheers,
Andrew