[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
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