Change nil :fx value to warning instead of error

Middle ground fix for #737. I don't think silence on nil is OK esp in
the context of this error message being in place for some time. However
it needs to be changed to a warning so that users are not misled into
thinking that nil is unsupported - if you intended it, then it is fine.

src/re_frame/fx.cljc

@@ -108,7 +108,7 @@
   :fx
   (fn [seq-of-effects]
     (if-not (sequential? seq-of-effects)
-      (console :error "re-frame: \":fx\" effect expects a seq, but was given " (type seq-of-effects))
+      (console :warn "re-frame: \":fx\" effect expects a seq, but was given " (type seq-of-effects))
       (doseq [[effect-key effect-value] (remove nil? seq-of-effects)]
         (when (= :db effect-key)
           (console :warn "re-frame: \":fx\" effect should not contain a :db effect"))