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

[JaviMerino]

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.

[sinkap]

The exception is left generic because, it not only the pivot that could cause the "no plot" error, it could be that the given set of filters applied lead to no output. But I agree, that there should be a check on the fact that the pivot is missing from the data. Will look into adding this.

[JaviMerino]

I'm not saying that we should change this exception, I agree that this particular exception should be generic. But for this particular case, the error message can be clearer.