devX: fail fast on interrupts with no checkpointer

[benjamincburns]

Problem

Interrupts only work when a checkpointer is set.

Expected behavior

When a graph doesn't have a checkpointer set and a node interrupts, we should throw a useful error.

Current behavior

Repeated calls to invoke or stream behave as though the graph was interrupted with a checkpointer present. It's only if the user happens to call getState to check for interrupts (not included in our examples/docs, IIRC) that they'll see a helpful error (No checkpointer set.).