added exceptions in insitumpi

Created by: JasonRuonanWang

The insitumpi engine will return successfully when there is only writer (or reader) launched. This is a little confusing when the MPI environment is getting complicated, for example, running cheetah workflow on Summit. In this case it's very hard for users to directly tell whether the writers and readers are launched using correct split MPI communicators. So even if they are not launched correctly, both the writers and readers are still going to finish successfully, but of course in this case data is not going to be correct at all. So it's better to throw an exception when there is no writers or no readers launched.