Through some integration testing checks, it looks like it's possible
that connecting to a new instance of the Dart VM may fail even after the
Dart VM instance has been discovered.

This means that _getDartVm should be checked in more than just one
place.  This changes the function to return a null value in the event of
a 'well-known' exception (`HttpException` or `TimeoutException`
specifically).

As a result, code now calling _getDartVm checks for null and handles it
as appropriate, and the DartVm RPC calls are all updated with
consistent timeout params.