Clearer error message if a dead channel appears in the waveform extractor.
Clearer error message if a dead channel appears in the waveform extractor.

kwikteam/phy,rossant/phy,kwikteam/phy,nsteinme/phy,kwikteam/phy,nippoo/phy,nsteinme/phy,nippoo/phy,rossant/phy,rossant/phy