Added a new Error Message for Missing Function Name.

The program:

function(){}

...now produces:
	SyntaxError: Function statements require a valid function name.

...instead of:
	SyntaxError: Unexpected Token (

Bug: v8:3698, v8:6513
Change-Id: I3c12dfcfe80b94209aa9af434ae1d212970cf362
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1500914
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60376}