getUnfHist throws a proper exception when missing observables

Unfolding/bin/getUnfHist.cc

@@ -58,6 +58,9 @@ void getUnfHist
     vector<unique_ptr<Filler>> fillers;
     for (const auto obs: observables)
         fillers.push_back(obs->getFiller(flow));
+    if (observables.empty())
+        BOOST_THROW_EXCEPTION( invalid_argument("You should provide at least one observable "
+                                                "(e.g. `InclusiveJet::PtY`)") );
 
     cout << "Setting variations" << endl;
     DistVariation::isMC = isMC;