Return a better error to the user when the GPO Portal is offline, and we cannot get their project list, which causes create_instance to fail.
Return a better error to the user when the GPO Portal is offline, and
we cannot get their project list, which causes create_instance to fail.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome