Bugfix: adding integer to string literal is not concatenation.

opm/parser/eclipse/Deck/SCHEDULESection.cpp

@@ -31,7 +31,7 @@ namespace Opm {
         if (timestep < m_decktimesteps.size()) {
             return m_decktimesteps[timestep];
         } else {
-            throw std::out_of_range("No DeckTimeStep in ScheduleSection for timestep " + timestep);
+            throw std::out_of_range("No DeckTimeStep in ScheduleSection for timestep " + std::to_string(timestep));
         }
     }