Better error when Plots is not loaded

[fonsp]

If you load a package that implements plot recipes, and call one of its plot functions without loading Plots.jl, the error is unhelpful:

The error should tell me to install Plots.jl and load it with using Plots.

[BeastyBlacksmith]

Currently I don't see an easy way of doing this. If we add a fallback method here, we override the fallback method of Plots.

[BeastyBlacksmith]

now that I read it again, GraphRecipes could warn at using time