fix(emacs/lean-server): check buffer-modified-p before create a process

fix #57

src/emacs/lean-server.el

@@ -80,6 +80,8 @@
 ;; ==============================
 (defun lean-server-create-process ()
   "Create lean-server process."
+  (when (buffer-modified-p)
+    (error "Please save the buffer before start lean-server."))
   (let ((process-connection-type nil)
         (lean-server-process
          (start-process lean-server-process-name