improve error message in `make-txexpr`

typed/txexpr/main.rkt

@@ -51,7 +51,15 @@
   (define result (cons tag (append (if (empty? attrs) empty (list attrs)) elements)))
   (if (txexpr? result)
       result
-      (error 'make-txexpr "This can't happen")))
+      (error 'make-txexpr
+             (cond
+               [(not (txexpr-tag? tag))
+                (format "This is not a txexpr-tag: ~v" tag)]
+               [(not (txexpr-attrs? attrs))
+                (format "This is not a list of txexpr-attrs: ~v" attrs)]
+               [(not (txexpr-elements? elements))
+                (format "This is not a list of txexpr-elements: ~v" elements)]
+               [else ""]))))
 
 
 (define/typed (txexpr->values x)