[parser] implement error reporting for Scanner errors

test/message/unicode-escape-invalid.out

+# Copyright 2016 the V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+*%(basename)s:5: SyntaxError: Unclosed Unicode escape sequence --- Expected `}`
+print("\u{FFYZ}");
+            ^
+SyntaxError: Unclosed Unicode escape sequence --- Expected `}`

[adamk, 2016/03/18 18:21:55]

As mentioned elsewhere, in this context just saying "Invalid unicode escape
sequence" or "Invalid hexadecimal escape sequence" seems fine, since the error
is the "Y", not the lack of a "}".

[caitp (gmail), 2016/03/18 19:00:36]

Acknowledged.

[caitp (gmail), 2016/03/21 16:15:03]

well, if you think about it, the scanner is in a state where if the next input
isn't a hex digit, it must be a `}` --- as far as the state machine is
concerned, it really is an unclosed unicode escape sequence.

But alright, I've removed the message, maybe it will be helpful to put back
later on.