Throw empty error from V8PromiseCustom

Blink and the inspector cannot deal with a stack overflow exception
thrown from V8PromiseCustom.
As a workaround, V8PromiseCustom throws an empty error when some exception
is thrown.

BUG=345088

Review URL: https://codereview.chromium.org/194713006

git-svn-id: svn://svn.chromium.org/blink/trunk@169028 bbb929c8-8fbe-4397-9dbb-9b2b20218538