Improve error message 'cannot perform concat operation between string and string' due to implicit coercion

[varungandhi-src]

So when you do something like:

import (
    lua "github.com/yuin/gopher-lua"
    luar "layeh.com/gopher-luar"
)

type S struct {
  A *lua.LString
}

func TestX(t *testing.T) {
	luaState := lua.NewState()
	defer luaState.Close()
	lstring := lua.LString("hello")
	s := S{A: &lstring}
	luaState.SetGlobal("s", luar.New(luaState, &s))
	if err := luaState.DoString(`return (s.A .. "!")`); err != nil {
		panic(err)
	}
}

This triggers an error cannot perform concat operation between string and string.

I wasn't sure if sub-standard error messages were considered bugs, so I've erred on the side of opening a new discussion instead of a bug report