Improve error message when last flow form is a vector

[Andre0991]

state-flow won't compile this:

(defflow foo
  (flow "consuming `installments-loan.new`"
    [:let [as-of #nu/time "2020-01-07T13:00:00Z"]]))

It gets this error:

Unexpected error (AssertionError) macroexpanding cats.core/do-let at (postman/postman/events/update_pv_for_closing_bill.clj:50:7).
Assert failed: Last argument of do-let must not be a vector
(not (vector? (last forms)))

It would be nice if it allowed such forms.

It makes no sense for the test, but it's useful for interactive development.

[dchelimsky]

A flow must return a flow/step, and we don't want to change that semantic. We can, however, improve the error message. Leaving this open until we do.

[Andre0991]

Hummm, yeah, that makes sense. Thanks.

[dchelimsky]

Addressed in PR #128