services: Never throw when stopping mount-may-fail file systems.

This is a followup to 7c27bd115b14afd142da7684cc349369965f9eab.

* gnu/services/base.scm (file-system-shepherd-service): Catch
'system-error from ‘umount’ call when FILE-SYSTEM is marked as
mount-may-fail.

Change-Id: I2234f8da320b43089f4ee058cad8608ce9c078f8

gnu/services/base.scm

@@ -448,7 +448,11 @@ upon boot."
                       ;; Make sure PID 1 doesn't keep TARGET busy.
                       (chdir "/")
 
-                      (umount #$target)
+                      #$(if (file-system-mount-may-fail? file-system)
+                            #~(catch 'system-error
+                                (lambda () (umount #$target))
+                                (const #f))
+                            #~(umount #$target))
                       #f))
 
             ;; We need additional modules.