Update METFixEE2017 jer(nominal) variation
Following https://github.com/cms-nanoAOD/nanoAOD-tools/issues/221 : removed the workaround from https://gitlab.cern.ch/cp3-cms/bamboo/merge_requests/72 , and updated the reference file.
I also added an error message in case no JEC tag is present, but something that depends on it is needed (otherwise this gives more obscure error messages later on).