Improve some printouts.

Specifically the one from ,rr.  Also make ,switch syntax error more
friendly.  Also, test for the recent xrepl fix, including its printout.
(cherry picked from commit e311de0522)

collects/tests/xrepl/xrepl.rkt

@@ -188,5 +188,18 @@
   json/main> «,sh echo $F»
   @|collects|/json/main.rkt
   json/main> «,top»
+  -> «(display-to-file "#lang racket\n(provide x)\n(define x 1)\n"
+                       "xrepl-test.rkt" #:exists 'truncate)»
+  -> «,rr "xrepl-test.rkt"»             ⇒ first load
+  ; requiring "xrepl-test.rkt"
+  -> «x»
+  1
+  -> «(display-to-file "#lang racket\n(provide x)\n(define x 2)\n"
+                       "xrepl-test.rkt" #:exists 'truncate)»
+  -> «,rr xrepl-test.rkt»
+  ; reloading "xrepl-test.rkt"
+  -> «,rm xrepl-test.rkt»
+  -> «x»
+  2
   -> «,ex»
   @||})

collects/xrepl/xrepl.rkt

@@ -110,15 +110,18 @@
              [best (if (< (string-length best) (string-length x)) best x)])
         best)))
   (define (get-prefix* path)
-    (define x (path->string path))
+    (define x (if (string? path) path (path->string path)))
     (define y (->relname path))
     (if (equal? x y)
       (format "~s" (choose-path x))
       (regexp-replace #rx"[.]rkt$" y "")))
+  (let loop ([mod mod])
     (match mod
       [(? symbol?)                 (symbol->string mod)]
-    [(list 'quote (? symbol? s)) (format "'~a" s)]
-    [_ (get-prefix* mod)]))
+      [(list 'quote (? symbol? s)) (format "'~a" (loop s))]
+      [(list 'file (? string? s))  (loop (string->path s))]
+      [(or (? path?) (? string?))  (get-prefix* mod)]
+      [_ (error 'xrepl "internal error; ~v" mod)])))
 
 (define (here-source) ; returns a path, a symbol, or #f (= not in a module)
   (variable-reference->module-source
@@ -749,14 +752,15 @@
     (for ([mod (in-list last-rr-modules)])
       (define resolved ((current-module-name-resolver) mod #f #f #f))
       (define path     (resolved-module-path-name resolved))
+      (define disp     (module-displayable-name mod))
       (if (hash-ref rr-modules resolved #f)
         ;; reload
-        (begin (printf "; reloading ~a\n" path)
+        (begin (printf "; reloading ~a\n" disp)
                (parameterize ([current-module-declare-name resolved])
                  (load/use-compiled path)))
         ;; require
         (begin (hash-set! rr-modules resolved #t)
-               (printf "; requiring ~a\n" path)
+               (printf "; requiring ~a\n" disp)
                ;; (namespace-require mod)
                (eval #`(require #,mod)))))))
 
@@ -1063,7 +1067,7 @@
       (current-namespace-name name)
       (current-namespace (car (hash-ref namespaces name)))))
   (define (syntax-error)
-    (cmderror "syntax error, see ,help switch-namespace"))
+    (cmderror "syntax error, see \",help ~s\"" (current-command)))
   (match (getarg 'sexpr 'list)
     [(list) (cmderror "what do you want to do?")]
     [(list '?)           (list-namespaces)]