Consider giving a error message instead of crashing when linked to ROOT without TMinuit2 support

[karuboniru]

When GENIE is linked against root without Minuit2 built in, running specific model will fail due to

Generator/src/Physics/Multinucleon/EventGen/MECGenerator.cxx

Line 1310 in 1e12986

ROOT::Math::Minimizer * min = ROOT::Math::Factory::CreateMinimizer("Minuit2");

will get nullptr, program will crash with SIGSEGV. This can be confusing.