The existance of an `AllowJavascriptExecutionDebugOnly` scope in
`Isolate::ReportPendingMessages()` indicates that the API supports
running arbitrary JS code in a `AddMessageListener` callback.

Currently, this can fail in debug mode: The
`!isolate->external_caught_exception()` condition is checked when
entering API methods inside such a handler. However, if there is
a verbose `TryCatch` active when the exception occurs, this
check fails, and when calling `ToString()` on the exception object
leaves a pending exception itself, the flag is re-set to `true`.

Fix this problem by clearing the flag and the pending exception if
there was one during `ToString()`. This matches the code a few lines
up in `messages.cc`, so the exception state is now consistent
during the callback.

This currently makes a Node.js test fail in debug mode
(`parallel/test-error-reporting`).

Bug: node:7144
Bug: node:17016
Change-Id: I060d00fea3e9a497f4df34c6ff8d6e29ebe96321
Reviewed-on: https://chromium-review.googlesource.com/718096
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49466}