file-systems: Separate ENOENT catching from ext2 superblock reads.

* gnu/build/file-systems.scm (ENOENT-safe): New procedure.
(read-ext2-superblock*): Rewrite in terms of it.

gnu/build/file-systems.scm

@@ -167,12 +167,13 @@ (define (partition? major minor)
                      (loop (cons name parts))
                      (loop parts))))))))))
 
-(define (read-ext2-superblock* device)
-  "Like 'read-ext2-superblock', but return #f when DEVICE does not exist
-instead of throwing an exception."
+(define (ENOENT-safe proc)
+  "Wrap the one-argument PROC such that ENOENT errors are caught and lead to a
+warning and #f as the result."
+  (lambda (device)
     (catch 'system-error
       (lambda ()
-      (read-ext2-superblock device))
+        (proc device))
       (lambda args
         ;; When running on the hand-made /dev,
         ;; 'disk-partitions' could return partitions for which
@@ -182,7 +183,10 @@ (define (read-ext2-superblock* device)
               (format (current-error-port)
                       "warning: device '~a' not found~%" device)
               #f)
-          (apply throw args)))))
+            (apply throw args))))))
+
+(define read-ext2-superblock*
+  (ENOENT-safe read-ext2-superblock))
 
 (define (partition-predicate field =)
   "Return a predicate that returns true if the FIELD of an ext2 superblock is