[parser] implement error reporting for Scanner errors

test/message/unicode-escape-invalid.out

Index: test/message/unicode-escape-invalid.out
diff --git a/test/message/regress/regress-4829-1.out b/test/message/unicode-escape-invalid.out
similarity index 54%
copy from test/message/regress/regress-4829-1.out
copy to test/message/unicode-escape-invalid.out
index 61b72a5bd7aadea8477e26cc19991588ca4bc600..2bdd53881e40342d47eab84cc3c8205b41348c80 100644
--- a/test/message/regress/regress-4829-1.out
+++ b/test/message/unicode-escape-invalid.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
-tag(tag`\xyy`);
-       ^^^
-SyntaxError: Invalid or unexpected token
+*%(basename)s:5: SyntaxError: Invalid Unicode escape sequence
+print("\u{FFYZ}");
+            ^
+SyntaxError: Invalid Unicode escape sequence