Proceed with upload even if port can't be found.

This is a quick / partial fix for issue #1791: an inability to upload without an explicit port (e.g. for the AVRISP mkII or USBtinyISP). Currently, it simply allows the upload to proceed if the port can't be found, leaving the error (if any) to occur later. We probably also need to do a better job of catching that errors that do occur and showing better error messages for them. (It probably doesn't make sense to try to preemptively check for the existence of the port, since the logic that determines whether or not a port is required for upload is rather complicated and internal to the SerialUploader class.)

Cristian, do you think it's okay to merge this as is to start? Or does better error handling need to be included first? Or any other thoughts / suggestions?