fix error message (#9)

Author: johri21

fixtures/error1.lua

@@ -0,0 +1,5 @@
+local api = require("test")
+
+function main(input)
+    return api.error1(input)
+end

module.go

@@ -13,8 +13,9 @@ import (
 )
 
 var (
-	errFuncInput  = errors.New("lua: function input arguments must be of type lua.Value")
-	errFuncOutput = errors.New("lua: function return values must be either (error) or (lua.Value, error)")
+	errFuncInput   = errors.New("lua: function input arguments must be of type lua.Value")
+	errFuncOutput  = errors.New("lua: function return values must be either (error) or (lua.Value, error)")
+	errorInterface = reflect.TypeOf((*error)(nil)).Elem()
 )
 
 var builtin = make(map[reflect.Type]func(any) lua.LGFunction, 8)
@@ -94,12 +95,12 @@ func (g *fngen) generate() lua.LGFunction {
 		switch len(out) {
 		case 1:
 			if err := out[0]; !err.IsNil() {
-				state.RaiseError(err.String())
+				state.RaiseError(err.Interface().(error).Error())
 			}
 			return 0
 		default:
 			if err := out[1]; !err.IsNil() {
-				state.RaiseError(err.String())
+				state.RaiseError(err.Interface().(error).Error())
 				return 0
 			}
 

module_test.go

@@ -5,6 +5,7 @@ package lua
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"hash/fnv"
 	"strings"
@@ -29,6 +30,7 @@ func testModule() Module {
 	must(m.Register("enrich", enrich))
 	must(m.Register("batch", batch))
 	must(m.Register("error", errorfunc))
+	must(m.Register("error1", errorfunc1))
 	return m
 }
 
@@ -44,6 +46,10 @@ func errorfunc(v String) (String, error) {
 	return "", fmt.Errorf("error with input (%v)", v)
 }
 
+func errorfunc1(_ Table) (String, error) {
+	return "", errors.New("throwing error")
+}
+
 func hash(s String) (Number, error) {
 	h := fnv.New32a()
 	h.Write([]byte(s))
@@ -202,6 +208,17 @@ func TestErrorMessage(t *testing.T) {
 	assert.Contains(t, err.Error(), "error with input (roman)")
 }
 
+func TestErrorMessage1(t *testing.T) {
+	s, err := newScript("fixtures/error1.lua")
+	assert.NoError(t, err)
+
+	_, err = s.Run(context.Background(), map[string]string{
+		"test1": "default",
+	})
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "throwing error")
+}
+
 func TestEnrichComplexTable(t *testing.T) {
 	s, err := newScript("fixtures/enrich.lua")
 	assert.NoError(t, err)