Error message when not having a backend confusing.

[ilyagr]

I found the error message "ERROR: Function dropdown was about to overflow: check the signature" very confusing when trying to use StatsPlots.dataviewer (JuliaPlots/StatsPlots.jl#370 ). It seems my mistake was actually that I didn't import Interact before using StatsPlots.dataviewer.

Perhaps the message should say something like

"Backend $backend doesn't implement the function $op. Perhaps you haven't imported Interact?".

I'm not sure whether this message would be appropriate in all cases in src/defaults.jl:9, or only when the backend is the DummyBackend.