improve error message when last arg to flow is a vector

[dchelimsky]

This PR addresses issue #113

Before

(flow "description" [x (state/get-state)])
;; Unexpected error (AssertionError) macroexpanding cats.core/do-let at (....).
;; Assert failed: Last argument of do-let must not be a vector
;; (not (vector? (last forms)))

After

(flow "description" [x (state/get-state)])
;; Syntax error macroexpanding state-flow.core/flow at (...).
;; The last argument to flow must be a flow/step, not a binding vector.

[dchelimsky]

Thanks @philomates