trappy plotter should throw an appropriate message if it's asked to pivot on something that's not in the trace

trappy.ILinePlot(trace, signals["sched_switch:next_pid"], pivot="cqu").view() throws back an exception like "no plots for the given constraints". While it is true, it could be a bit more specific. In this case it's obvious that the pivot doesn't exist, but if we gave back something like "You're trying to pivot on "cqu" which is not present in sched_switch:next_pid" it would be a lot more clear that I typoed "cpu".

Reported by @deggeman.