Require Debug+Display, not Error

In ICU4X we're moving towards `no_std` support, and this means that _by default_ the `Error` trait is not implemented. It's possible to explicitly enable the `"std"` feature and have it be enabled, however it seems like it's unnecessary to require `Error` on the error associated types since none of `Error`s functionality is actually used — perhaps we should just require `Debug+Display`, or maybe even just one or none of those traits. In general we should require the traits we need.