Improve error message for duplicate parameters

Improve error message for duplicate parameters

Duplicate parameters are banned both overall in strict mode and also
in arrow functions. Our error message for both cases blamed strict
mode, which is confusing. This patch fixes the message to point to
arrow functions as a possible source as well.

R=wingo, adamk
LOG=N

Review URL: https://codereview.chromium.org/1236863008

Cr-Commit-Position: refs/heads/master@{#29662}