ui: Improve reporting of 'system-error' exceptions.

* guix/ui.scm (call-with-error-handling): Change 'system-error' handler
  to display the error message as it was raised.

guix/ui.scm

@@ -194,9 +194,9 @@ (define (call-with-error-handling thunk)
     ;; Catch EPIPE and the likes.
     (catch 'system-error
       thunk
-      (lambda args
-        (leave (_ "~a~%")
-               (strerror (system-error-errno args)))))))
+      (lambda (key proc format-string format-args . rest)
+        (leave (_ "~a: ~a~%") proc
+               (apply format #f format-string format-args))))))
 
 (define (read/eval str)
   "Read and evaluate STR, raising an error if something goes wrong."