Fix hexadecimal escape sequence validation in strings.

Fix hexadecimal escape sequence validation in strings.

Give an error message when the sequence does not conform to the grammar.
Although the specification does not explicitly state that this is an
error, this is the behaviour of both JSC and V8.

Change-Id: I34d189f07628bc6cc40b13bfbb8d09bee7810ced
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>