Fail on startup when sources and targets aren't reachable

[colmsnowplow]

When provided with the correct set of env vars for authentication, the EventHub target will create a client.

However, it currently doesn't ping the endpoint or check that it can connect - so if the credentials themselves are invalid, we don't find out until we send some actual data.

It will be easier to manage deployments if the app fails on startup in this scenario - indeed some of the targets will do so. We should:

a) Verify what the behaviour is for all targets
b) For those targets whose clients don't do so already, instrument some behaviour to check the connection at creation of target
c) Do the same for the sources