`withClient` API can hit precondition

[FranzBusch]

Describe the bug

If the withGRPCClient API is called with an empty handleClient closure then it is possible to hit the The client has stopped and can only be started once. precondition in its internal state machine. The reason for this is that beginGracefulShutdown() is called before runConnections().

To reproduce

Write a test that does this

try await GRPCClient.withGRPCClient(....) {}

Expected behaviour

We should handle this gracefully

[glbrntt]

Oh –- it's not a precondition, just an error. Nevertheless this should be fixed.

[glbrntt]

This can't (easily) be fixed in a sensible way: if the body of withGRPCClient returns quickly and never uses the client then there's an inherent race here. Given this is a weird edge case I think the right move is to just improve the diagnostic.