Exit if requested upload file does not exist

It was fruitlessly sending to server, when it could detect the problem
up front without bothering the server.

confluent_client/bin/nodemedia

@@ -101,6 +101,10 @@ def detach_media(noderange, media):
 
 def upload_media(noderange, media):
     global exitcode
+    if not os.path.exists(media):
+        sys.stderr.write('Unable to locate requested file {0}\n'.format(
+            media))
+        sys.exit(404)
     session = client.Command()
     output = sq.ScreenPrinter(noderange, session)
     filename = os.path.abspath(media)