Add error-check for yield-in-pcall to traceback_pcall

luprex/core/cpp/traceback.hpp

@@ -25,7 +25,8 @@ int traceback_coroutine(lua_State *L);
 // Similar to lua_pcall, except that it automatically supplies 
 // traceback_coroutine as a message handler.  It also automatically
 // returns any error message.  Returns empty string if there's
-// no error.
+// no error.  Also checks for a yield inside a pcall, which is 
+// not allowed, and does an assert-fail in that case.
 //
 eng::string traceback_pcall(lua_State *L, int narg, int nret);