Fix use of invalidated iterator in the command map on exception

The command map can get mutated by command execution, so the iterators
can get invalidated (now that we use our curstom HashMap implementation,
all iterators are potentially invalidated by insert/removal)

src/command_manager.cc

@@ -456,7 +456,7 @@ void CommandManager::execute_single_command(CommandParameters params,
     catch (runtime_error& error)
     {
         throw runtime_error(format("{}:{}: '{}' {}", pos.line+1, pos.column+1,
-                                   command_it->key, error.what()));
+                                   params[0], error.what()));
     }
 }