change `get-preference' to treat read errors the same as other bad content

That is, return '(), but also now log an error.

collects/racket/file.rkt

@@ -354,6 +354,11 @@
                                      #f))))))])
         (let ([prefs (with-pref-params
                       (lambda ()
+                        (with-handlers ([exn:fail:read? (lambda (exn)
+                                                          (log-error 
+                                                           (format "error reading preferences: ~a"
+                                                                   (exn-message exn)))
+                                                          null)])
                           (if use-lock? 
                               (call-with-preference-file-lock
                                'get-preference
@@ -363,14 +368,19 @@
                                (lambda ()
                                  (with-input-from-file pref-file read))
                                lock-there)
-                            (with-input-from-file pref-file read))))])
+                              (with-input-from-file pref-file read)))))])
           ;; Make sure file content had the right shape:
           (if (and (list? prefs)
                    (andmap (lambda (x)
-                             (and (pair? x) (pair? (cdr x)) (null? (cddr x))))
+                             (and (pair? x) 
+                                  (symbol? (car x))
+                                  (pair? (cdr x)) 
+                                  (null? (cddr x))))
                            prefs))
               prefs
-              null)))))
+              (begin
+                (log-error "preference file content is not a list of symbol--value lists")
+                null))))))
   (let* ([fn (path->complete-path
               (or filename
                   (find-system-path 'pref-file)))]

collects/scribblings/reference/filesystem.scrbl

@@ -937,11 +937,13 @@ read an @elemref["old-prefs"]{old preferences file}, and then a
 collection, instead. If none of those files exists, the preference set
 is empty.
 
-The preference file should contain a symbol-keyed association list
-(written to the file with the default parameter settings).  Keys
+The preference file should contain a list of symbol--value lists
+written with the default parameter settings.  Keys
 starting with @racket[racket:], @racket[mzscheme:], @racket[mred:],
 and @racket[plt:] in any letter case are reserved for use by Racket
-implementers.
+implementers. If the preference file does not contain a list
+of symbol--value lists, an error is logged via @racket[log-error]
+and @racket[failure-thunk] is called.
 
 The result of @racket[get-preference] is the value associated with
 @racket[name] if it exists in the association list, or the result of