Gracefully handle unexpected syntax#36

## Brief `defn-spec` could be a superset of `defn`, but it isn't, and also not in a very informative manner: ``` orchestra.expound-test=> (defn-spec foo []) IllegalArgumentException find not supported on type: clojure.lang.Keyword clojure.lang.RT.find (RT.java:863) orchestra.expound-test=> (defn-spec foo [a]) IllegalArgumentException find not supported on type: clojure.lang.Keyword clojure.lang.RT.find (RT.java:863) orchestra.expound-test=> (defn-spec foo [a any?]) IllegalArgumentException find not supported on type: clojure.lang.Keyword clojure.lang.RT.find (RT.java:863) orchestra.expound-test=> (defn-spec foo any? [a any?]) orchestra.expound-test/foo ``` IOW, `find not supported on type: clojure.lang.Keyword` is not the best message one can find when writing a defn-spec using "muscle memory" from defn. ## Fix I'd suggest at least one of the following: * If specs are omitted, they are assumed to be `any?`, possibly emitting a warning. All of this could be configurable behavior. * Specs are required, otherwise the situation is precisely detected and informed of, in plain English: * `No :ret spec supplied. Did you forget to add a spec after the function name?` * `No :args spec supplied for the last parameter. Did you forget to add a spec after the last parameter?` WDYT? Cheers - Victor