Implement custom reconciler errors

[aran]

In writing a custom reconciler for a data type to use with autosurgeon(with=..., I want to call a method that returns a Result.

To return an error from that Result, I need an API to allow me to construct an instance of Reconciler::Error. Currently that type is std::error::Error + From<StaleHeads>. Adding something like From<some::error::Type> or adding a method to Reconciler like error(some::error::Type) would allow me to communicate errors back to the reconcile caller.

[alexjg]

I wonder if we should just not have a generic type for the error but instead have it always be ReconcileError as that's what it is in practice.

[aran]

That would work well, especially if we could add another variant for some kind of other error.