There is _a lot_ of error types in the `amount` module. Move them to a
separate `error` module.
Add a bunch of `pub(super)` to keep things private to the `amount`
module.
Eventually we will want to close all these errors.
In preparation for splitting the error types out of `amount.rs` into
their own file move the `amount.rs` file to `amount/mod.rs`.
File move only, no other changes.