lexer: ErrUnacceptableRune and ErrNotImplemented

Signed-off-by: Alejandro Mery <amery@jpi.io>

lexer/error.go

@@ -1,6 +1,7 @@
 package lexer
 
 import (
+	"errors"
 	"fmt"
 	"strings"
 )
@@ -9,6 +10,14 @@ var (
 	_ error = (*Error)(nil)
 )
 
+var (
+	// ErrUnacceptableRune indicates the read rune isn't acceptable in the context
+	ErrUnacceptableRune = errors.New("rune not acceptable in context")
+
+	// ErrNotImplemented indicates something hasn't been implemented yet
+	ErrNotImplemented = errors.New("not implemented")
+)
+
 // Error represents a generic parsing error
 type Error struct {
 	Filename string