discovery: 'scheme-files' returns '() for a non-accessible directory.

Fixes a regression introduced in
d27cc3bfaafe6b5b0831e88afb1c46311d382a0b.

Reported by Ricardo Wurmus <rekado@elephly.net>.

* guix/discovery.scm (scheme-files): Catch 'scandir*' system errors.
Return '() and optionally raise a warning upon 'system-error'.
* tests/discovery.scm ("scheme-modules, non-existent directory"): New
test.

guix/discovery.scm

@@ -38,7 +38,8 @@
 
 (define* (scheme-files directory)
   "Return the list of Scheme files found under DIRECTORY, recursively.  The
-returned list is sorted in alphabetical order."
+returned list is sorted in alphabetical order.  Return the empty list if
+DIRECTORY is not accessible."
   (define (entry-type name properties)
     (match (assoc-ref properties 'type)
       ('unknown
@@ -67,7 +68,15 @@ returned list is sorted in alphabetical order."
                        (else
                         result))))))
               '()
-              (scandir* directory)))
+              (catch 'system-error
+                (lambda ()
+                  (scandir* directory))
+                (lambda args
+                  (let ((errno (system-error-errno args)))
+                    (unless (= errno ENOENT)
+                      (warning (G_ "cannot access `~a': ~a~%")
+                               directory (strerror errno)))
+                    '())))))
 
 (define file-name->module-name
   (let ((not-slash (char-set-complement (char-set #\/))))

tests/discovery.scm

@@ -32,6 +32,10 @@
     ((('guix 'import _ ...) ..1)
      #t)))
 
+(test-equal "scheme-modules, non-existent directory"
+  '()
+  (scheme-modules "/does/not/exist"))
+
 (test-assert "all-modules"
   (match (map module-name
               (all-modules `((,%top-srcdir . "guix/build-system"))))