If 'servername' is not provided, 'ERR_MISSING_ARGS is thrown.
If 'servername' is not a string, 'ERR_INVALID_ARG_TYPE' is thrown.
If 'servername' is an IP address, 'ERR_INVALID_ARG_VALUE' is thrown, since
literal IPv4 and IPv6 addresses are not permitted in SNI.

Fixed API documentation ('hostname' -> 'servername').

Also removed a redundant error 'ERR_TLS_REQUIRED_SERVER_NAME'.