Make the "duplicate pattern variable" message more informative

Having the `sym` and `pattern-vars` info makes it possible to guess
which expression the error concerns.

lib/_syntax-pattern.scm

@@ -117,7 +117,7 @@
             (else
              (let ((x (assq sym pattern-vars)))
                (if x
-                   (error "duplicate pattern variable")
+                   (error "duplicate pattern variable: " sym pattern-vars)
                    (let ((index (length pattern-vars)))
                      (cont (vector (syn#pattern-var)) ;; TODO: replace with '#(0) to allow sharing?
                            (cons (cons sym (cons index rank))