src: handle TryCatch with empty message
This bug needs a test case with a high goldilocks factor to trigger
but the synopsis is that v8::TryCatch::Message()
returns an empty
handle when the TryCatch is declared at a time when an exception is
already pending.
We now recompute the message inside node::ReportException()
and
all is well again.
Fixes: https://github.com/nodejs/node/issues/8854
CI: https://ci.nodejs.org/job/node-test-pull-request/14850/
CI: https://ci.nodejs.org/job/node-test-pull-request/14851/
CI: https://ci.nodejs.org/job/node-test-pull-request/14861/ (minor test tweaks)