Better debugging info for when things break

[willtebbutt]

Relates to #80 -- FiniteDifferences cannot generally handle functions for which either the size the type of the output is a function of the value of the input. Given that these are fundamental limitations, rather than bugs, it makes sense to provide tooling to help users understand when this issues are encountered.

I propose to add a mode (somehow) that lets the user get easy access to the inputs are which the function will be evaluated by FiniteDifferences. This should be sufficient to make it easy for the user to figure out what's going on when things break.

It would be nice to add helpful warnings when standard things go wrong - eg. your function's output type is different for different evaluations of the function, which can easily happen in slightly unexpected ways, such as in #80 .