upgrade.sh: Doc, error message, and pedantic output improvements.
* Enhance docs to make it super clear p4d must be running at the start.
* Enhance error message for the specific case where the only preflight check to fail is the one about p4d not being up, and say that if it was manually stopped, that it should be started before calling the script again.
* Display the full command line of the call to verify_sdp.sh as made in upgrade.sh in the log, so the customer can run the same verify_sdp.sh command manually if they want.