I should have said :) I am writing backtracking algorithms (think something like a Sudoku solver where we fill in values by guessing), and an 'Error' is when we can deduce the Sudoku cannot be filled in, so we have to backtrack.

I find Rust's ? notation gives a very natural way of writing such algorithms. I don't care "why" filling in the Sudoku failed, particularly because the solver will typically fail millions of times a second, so I definately don't want constructing the error to be expensive in any way.