This updates the scanner to use the correct error message when it
encounters an octal escape sequence in a template literal. Previously,
the error message referred to strict mode, even when the template
literal was not in strict mode code.

Bug: v8:7502
Change-Id: I37bb1338cf796c471108bc10f35f824cdf3ce0b7
Reviewed-on: https://chromium-review.googlesource.com/945411Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51823}