Fail hard if SSL certs or keys cannot be read by user

[stanhu]

Description

Previously if an SSL cert or key could not be read, Puma would bind to
the configured SSL port but not accept any connections. The only
indication that something went awry is an obscure log message:

 #<Puma::MiniSSL::SSLError: OpenSSL error: error:1417A0C1:SSL routines:tls_post_process_client_hello:no shared cipher - 193>

We now fail hard with an exception if this happens to make it clear
that the permissions need to be fixed.

Relates to #1339

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[MSP-Greg]

@stanhu

Thanks for the PR, good catch/idea. Maybe add a method for file checking, something like:

      def check_file(file, desc)
        raise ArgumentError, "#{desc} file '#{file}' does not exist" unless File.exist? file
        raise ArgumentError, "#{desc} file '#{file}' is not readable" unless File.readable? file
      end

Also, perform the same update to the JRuby keystore file?

See MSP-Greg@a254765, I can push the commit to your branch.

[stanhu]

Works for me!