Improve errors for potential arrow functions

There are certain locations where arrow functions don't parse ```ts var x = window.foo || () => {} ``` In these instances, it would be nice if users could get a better error message as to avoid issues like #25897. Ideally this should not be an invasive change. * The change should be limited in scope. * The parser should continue erroring in those locations. * The change must not introduce change how existing valid code is parsed.