[blobstore_client] extra help if misinvoked

Output now looks like:

    $ bin/blobstore_client_console
   Usage: blobstore_client_console [options]
       -p, --provider PROVIDER
       -c, --config FILE

    Example config file:
   ---
   endpoint: http://1.2.3.4:25250
   user: agent
   password: agent
   bucket: resources