rPb757e5c30249

Use "MethodCallFuture" to move Diffusion Conduit exceptions to resolution time

Summary:
Depends on D21720. Ref T13666. See D21720 for additional discussion.

Use "MethodCallFuture", introduced in D21720, so that exceptions raised in "execute()" are thrown when the future is resolved, not when the future is created.

This makes exception behavior for clustered and non-clustered setups consistent, and chooses the intended (clustered) behavior in both cases, which currently deals with errors better.

Test…