In our internal code, we should only use pending exceptions. They will
be converted to scheduled exceptions on the API boundary.
Hence, the ErrorThrower just sets a pending exception; it should never
have to think about scheduled exceptions. The new
ScheduledErrorThrower inherits from ErrorThrower and reschedules any
pending exceptions in its destructor (turning them into scheduled
exceptions).
In some situations, there might already be a scheduled exception, e.g.
when calling other API methods (v8::Value::Get). In this case, the
ErrorThrower should also not set another pending exception. For the
reasons mentioned above, this can only be handled in the
ScheduledErrorThrower, which is used the API methods.

This fixes one DCHECK failure and one TODO about scheduled exceptions
if no instance can be created, because the start function throws.

R=mtrofin@chromium.org, mstarzinger@chromium.org
BUG=v8:6232,chromium:736256

Change-Id: I4905be04c565df9495de18fb26adbb5c05d193d2
Reviewed-on: https://chromium-review.googlesource.com/548641
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46314}