repl: make 'Unexpected token' errors recoverable

Fix the regexp used to detect 'Unexpected token' errors so that they can
be considered as recoverable. This fixes the following use case:

> var foo = 'bar \
... baz';
undefined
> foo
'bar baz'
>

Fixes #8874

Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
PR-URL: https://github.com/joyent/node/pull/8875