Plugin should fail with a configuration error is the number of threads is less than 2

[robbavey]

As described in this issue, starting the plugin with a thread count less than 2 is problematic, and should be explicitly disallowed.

When setting the number of threads to 1, the plugin appears to be working correctly, but will not process any events from any event hubs.

We should explicitly raise a ConfigurationError when the number of thread is set to less than 2 and prevent the plugin from starting