Add an error message for magit-display-process.

This message is more descriptive than the default
when Magit hasn't run a Git command.

magit.el

@@ -1397,6 +1397,8 @@ FUNC should leave point at the end of the modified region"
 (defun magit-display-process ()
   "Display output from most recent git command"
   (interactive)
+  (unless (get-buffer magit-process-buffer-name)
+    (error "No Git commands have run"))
   (display-buffer magit-process-buffer-name))
 
 ;;; Mode