Add error

Author: antonmedv

error.go

@@ -0,0 +1,20 @@
+package expr
+
+import (
+	"fmt"
+	"strings"
+)
+
+type syntaxError struct {
+	message string
+	input   string
+	pos     int
+}
+
+func (e *syntaxError) Error() string {
+	snippet := ""
+	if len(e.input) > 0 {
+		snippet = fmt.Sprintf("\n%s\n%s", e.input, strings.Repeat("-", e.pos)+"^")
+	}
+	return e.message + snippet
+}