xgotext: fixed parsing of error types

cli/xgotext/fixtures/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/leonelquinteros/gotext"
@@ -78,6 +79,9 @@ func main() {
 	// redefine alias with fake struct
 	alias := Fake2{}
 	alias.Get("3")
+
+	err := errors.New("test")
+	fmt.Print(err.Error())
 }
 
 // dummy function

cli/xgotext/parser/golang.go

@@ -165,7 +165,7 @@ func (g *GoFile) checkType(rawType types.Type) bool {
 		return g.checkType(t.Elem())
 
 	case *types.Named:
-		if t.Obj().Pkg().Path() != "github.com/leonelquinteros/gotext" {
+		if t.Obj().Pkg() == nil || t.Obj().Pkg().Path() != "github.com/leonelquinteros/gotext" {
 			return false
 		}
 	default: