Change CreateTaskOrchestrationAsync tp throw OrchestrationAlreadyExistsException

[cgillum]

This is to allow exception handling code to catch a more specific exception and differentiate between unexpected errors and expected errors.

This is largely non-breaking because OrchestrationAlreadyExistsException already derives from InvalidOperationException.

Note that the IOrchestrationServiceClient interface already documents that the CreateTaskOrchestrationAsync methods should be throwing OrchestrationAlreadyExistsException, so this change fixes the AzureStorageOrchestrationService's implementation to follow the existing spec.

[davidmrdavid]

let's wait for CI

[cgillum]

Looks like the failing CI is new and not part of our normal CIs. I think we can safely ignore its results.

[davidmrdavid]

@cgillum: agreed. Surprised to see it running automatically on every PR, but that's fine since it's just tests.