[parser] implement error reporting for Scanner errors

test/message/regress/regress-4829-2.out

Index: test/message/regress/regress-4829-2.out
diff --git a/test/message/regress/regress-4829-2.out b/test/message/regress/regress-4829-2.out
index ec281d90d73522cb15b37d5ef3119ad1f7be3316..c8272b4db0a496ef6cc4aa36afbd502dac5f459e 100644
--- a/test/message/regress/regress-4829-2.out
+++ b/test/message/regress/regress-4829-2.out
@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-*%(basename)s:7: SyntaxError: Invalid or unexpected token
+*%(basename)s:7: SyntaxError: Invalid hexadecimal escape sequence
 `${tag`\xyy`}`;
-      ^^^
-SyntaxError: Invalid or unexpected token
+       ^^^^
+SyntaxError: Invalid hexadecimal escape sequence