Have render-compare/package-derivations provide invalid param info

So that this can be used by the qa-frontpage.

This should be improved and generalised.

guix-data-service/web/compare/controller.scm

@@ -649,7 +649,24 @@
              mime-types)
         ((application/json)
          (render-json
-          '((error . "invalid query"))))
+          `((error . "invalid query")
+            (query_parameters
+             .
+             ,(map
+               (match-lambda
+                 ((name . val)
+                  (cons
+                   name
+                   (if (invalid-query-parameter? val)
+                       `((invalid
+                          . ,(with-output-to-string
+                               (lambda ()
+                                 (sxml->html
+                                  (invalid-query-parameter-message
+                                   val)))))
+                         (value . ,(invalid-query-parameter-value val)))
+                       val))))
+               query-parameters)))))
         (else
          (letpar& ((systems
                     (with-thread-postgresql-connection