sed -i 's/\(\bfn .*) \)%\(.*{\)$/\1!\2/g' $(find . -name "*.zig")

the literal translation of `%T` to this new code is `error!T`.
however this would not take advantage of error sets. It's
recommended to generally have all your functions which return possible
errors to use error set inference, like this:

fn foo() !void {

}

then you can return void, or any error, and the error set is inferred.

// TODO this is an explicit cast and should actually coerce the type
   erorr set casting