Handle OpenSSL::SSL:SSLError

We use a server config where Webrick also handles HTTPS. When a connection is aborted, an exception is logged. For example:

$ nc localhost 8443
$ cat server.log
2021-05-26T11:52:34  [E] <OpenSSL::SSL::SSLError> SSL_accept SYSCALL returned=5 errno=0 state=before SSL initialization
        /usr/share/ruby/webrick/server.rb:299:in `accept'
        /usr/share/ruby/webrick/server.rb:299:in `block (2 levels) in start_thread'
        /usr/share/ruby/webrick/utils.rb:263:in `timeout'
        /usr/share/ruby/webrick/server.rb:297:in `block in start_thread'
        /usr/share/gems/gems/logging-2.3.0/lib/logging/diagnostic_context.rb:474:in `block in create_with_logging_context'

This comes from:
https://github.com/ruby/webrick/blob/3515081a51b91b730267ba2b224039ecfbf8bd7b/lib/webrick/server.rb#L252-L277

It does look like various basic network errors are caught and ignored, but SSL errors can fall in the same category (such as this one). Not everything should be logged at the error level. I'm not sure how it should exactly be dealt with (or I'd submit a patch instead of a PR) so I'm looking for input on this.

Start in lib/webrick/server.rb around lines 252-277 and reproduce the aborted HTTPS connection with nc against the Webrick HTTPS listener on port 8443. Compare the existing handling of basic network errors with this OpenSSL::SSL::SSLError and establish the expected logging behavior for benign connection aborts; done means that behavior is implemented and verified.