Improve dynamic require of requires analysis

Skip tests when unavailable.

Issue error, if any, on the command thread so message is reflected up
to Emacs for users to see.

racket/commands/requires.rkt

@@ -61,36 +61,39 @@
     ;; derivation-parser: error on token #2: <local-value,
     ;;#<syntax:/usr/share/racket/collects/syntax/parse/private/lib.rkt:63:55
     ;;-string>>
-    (when (version<=? "8.14" (version))
-      (check-equal?
-       (requires/trim p)
-       '((delete 174 17)
-         (replace
-          129
-          44
-          "(require net/url\n         (combine-in net/url\n                     racket/format))")
-         (replace 39 42 "(require net/uri-codec\n         net/url)")))
-      (check-equal?
-       (requires/base p)
-       '((delete 174 17)
-         (replace
-          129
-          44
-          "(require (for-syntax racket/base)\n         net/url\n         (combine-in net/url\n                     racket/format)\n         racket/list\n         racket/match)")
-         (replace 39 42 "(require net/uri-codec\n         net/url)"))))))
+    (cond
+      [(and show-requires
+            (version<=? "8.14" (version)))
+       (check-equal?
+        (requires/trim p)
+        '((delete 174 17)
+          (replace
+           129
+           44
+           "(require net/url\n         (combine-in net/url\n                     racket/format))")
+          (replace 39 42 "(require net/uri-codec\n         net/url)")))
+       (check-equal?
+        (requires/base p)
+        '((delete 174 17)
+          (replace
+           129
+           44
+           "(require (for-syntax racket/base)\n         net/url\n         (combine-in net/url\n                     racket/format)\n         racket/list\n         racket/match)")
+          (replace 39 42 "(require net/uri-codec\n         net/url)")))]
+      [else
+       (displayln "Skipping requires analysis tests")])))
 
 ;;; analysis by macro-debugger/analysis/check-requires
 
-(define (error-thunk)
-  (error 'requires
-         "Cannot work until you `raco pkg install macro-debugger-lib`"))
-
 (define show-requires
   (safe-dynamic-require 'macro-debugger/analysis/check-requires
                         'show-requires
-                        error-thunk))
+                        (λ () #f)))
 
 (define (analyze path-str)
+  (unless show-requires
+    (error 'analyze-requires
+           "Cannot work until you `raco pkg install macro-debugger-text-lib`"))
   (define-values (base name _) (split-path (string->path path-str)))
   (parameterize ([current-load-relative-directory base]
                  [current-directory base])