Change error prefix in C code from "ERROR" to "PANIC"

That's what Lua use for this kind of error.

luapak/wrapper_tmpl.lua

@@ -299,7 +299,7 @@ static int pushargs (lua_State *L) {
 int main (int argc, char *argv[]) {
   lua_State *L = luaL_newstate();
   if (L == NULL) {
-    lua_writestringerror("%s\n", "ERROR: not enough memory");
+    lua_writestringerror("%s\n", "PANIC: not enough memory");
     return EXIT_FAILURE;
   }