error_connection_closed to be confused with error_connect_closed

[tisba]

I see both errors from time to time which is a bit confusing.

error_connection_closed: https://github.com/processone/tsung/blob/master/src/tsung/ts_client.erl#L844
error_connect_closed: https://github.com/processone/tsung/blob/master/src/tsung/ts_client.erl#L891

According to https://tsung.readthedocs.org/en/latest/errorslist.html#error-connect-errorname (and that's correct IMO), error_connect_ prefixed errors indicate an error while establishing a connection to the server. So, error_connect_closed is correct.

Whereas error_connection_closed is reported when the connection is closed while sending data to the server. In this case the connection is already established and it should be prefixed with error_send_ (https://tsung.readthedocs.org/en/latest/errorslist.html#error-send-errorname).

So I propose to rename error_connection_closed to error_send_connection_closed. I'm happy to provide a pull request.

[nniclausse]

I'm ok with this