better message

src/tools/NeighborList.cpp

@@ -45,7 +45,8 @@ NeighborList::NeighborList(const vector<AtomNumber>& list0, const vector<AtomNum
   if(!do_pair) {
     nallpairs_=nlist0_*nlist1_;
   } else {
-    plumed_massert(nlist0_==nlist1_,"when using PAIR option, the two groups should have the same number of elements");
+    plumed_assert(nlist0_==nlist1_) << "when using PAIR option, the two groups should have the same number of elements\n"
+                                    << "the groups you specified have size "<<nlist0_<<" and "<<nlist1_;
     nallpairs_=nlist0_;
   }
   initialize();