Handling extension errors

[j-fu]

Regarding #106 and #111

I generally would prefer the approach with less boilerplate.

My current pattern to handle this situation is a bit different from the one implemented here in the moment.

function to_be_extended(args...; kwargs...)
   error("Please load the extension Xext")
end

In the extension:

function to_be_extended(MyType, ...)
...
end