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Such an error message will not appear on rubies version 2.1 and higher. Socket.tcp_server_sockets method used by webrick to start listening for connections [1] now silently discards errors as long as one address out of multiple is available for listening on [2].
In previous versions of ruby webrick was handling these errors and was logging them as warnings [3]. As 2.0.0 is the last version of ruby that has this issue, I'd opt for leaving things as is.

[1] https://github.com/ruby/ruby/blob/ruby_2_1/lib/webrick/utils.rb#L75
[2] https://github.com/ruby/ruby/blob/ruby_2_1/ext/socket/lib/socket.rb#L452
[3] https://github.com/ruby/ruby/blob/ruby_2_0_0/lib/webrick/utils.rb#L90