Do not crash when unknown symbol is passed to “irf_shocks” option

Closes: #57
(cherry picked from commit 4b8290fa)

src/ParsingDriver.cc

@@ -1413,9 +1413,13 @@ ParsingDriver::option_symbol_list(string name_option)
     {
       vector<string> shocks = symbol_list.get_symbols();
       for (auto &shock : shocks)
+        {
+          if (!mod_file->symbol_table.exists(shock))
+            error("Unknown symbol: " + shock);
           if (mod_file->symbol_table.getType(shock) != SymbolType::exogenous)
             error("Variables passed to irf_shocks must be exogenous. Caused by: " + shock);
         }
+    }
 
   if (name_option.compare("ms.parameters") == 0)
     {