guix system: Properly report Shepherd errors when upgrading services.

Fixes regression introduced in 8bf92e39.
Reported by myglc2 <myglc2@gmail.com> in <http://bugs.gnu.org/24135>.

* guix/scripts/system.scm (with-shepherd-error-handling): Rename 'body'
to 'mbody'.  Expand to a monadic procedure that runs MBODY.

guix/scripts/system.scm

@@ -236,11 +236,15 @@ (define-syntax-rule (warn-on-system-error body ...)
       (with-monad %store-monad
         (return #f)))))
 
-(define-syntax-rule (with-shepherd-error-handling body ...)
-  (warn-on-system-error
-   (guard (c ((shepherd-error? c)
-              (report-shepherd-error c)))
-     body ...)))
+(define-syntax-rule (with-shepherd-error-handling mbody ...)
+  "Catch and report Shepherd errors that arise when binding MBODY, a monadic
+expression in %STORE-MONAD."
+  (lambda (store)
+    (warn-on-system-error
+     (guard (c ((shepherd-error? c)
+                (report-shepherd-error c)))
+       (values (run-with-store store (begin mbody ...))
+               store)))))
 
 (define (report-shepherd-error error)
   "Report ERROR, a '&shepherd-error' error condition object."