improved error messages for special forms

[zevv]

This patch improves the error messages for special forms somewhat.

Below is a list with the wrong code, the old error message and the new error message:

(defn test [body msg_old msg_new]                                                                                      
  (def msg (try (eval body) ([err fib] err)))
  (if-not (= msg msg_old) (pp [body msg])))

(def foo @[1 2 3])

(test '(while true)    "expected at least 2 arguments"                                                                             
                       "expected at least 2 arguments to while")

(test '(var alice)     "expected at least 2 arguments"                                                                           
                       "expected at least 2 arguments to var") 

(test '(def bob)       "expected at least 2 arguments"                                                                           
                       "expected at least 2 arguments to def")

(test '(defn foo)      "(macro) expected integer key in range [0, 0), got 0"                                                                              
                       "(macro) expected integer key for tuple in range [0, 0), got 0")

(test '(set)           "expected 2 arguments"                                                                           
                       "expected 2 arguments to set")

(test '(set a)         "expected 2 arguments"                                                                           
                       "expected 2 arguments to set")

(test '(set (foo 5) 1) "expected 2 arguments"                                                                           
                       "expected 2 arguments to set")