Fix Jetpack connection error message

This references a closed ticket at https://ww.wp.xz.cn/support/topic/setup-jetpack-an-error-occurred-please-try-again/

A more helpful error message should be returned such as “Problem connecting to XML RPC. Please check your settings.”

I assume there is a hook connected to this step of the login process that can allow the install to fail gracefully with an internationalized message.

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