Add warning if someone tries attach with a file

Rather than bother the server, sort out the problem locally.

confluent_client/bin/nodemedia

@@ -63,6 +63,9 @@ def printerror(res, node=None):
 def attach_media(noderange, media):
     global exitcode
     session = client.Command()
+    if ':' not in media:
+        sys.stderr.write('Full URL required for attach')
+        sys.exit(1)
     resource = '/noderange/{0}/media/attach'.format(noderange)
     for res in session.update(resource, {'url': media}):
         printerror(res)