* types: add support for `PreconditionFailure` error message type
Following implementation at flemosr/tonic-richer-error.
* types: doc comments nits
* types: merge `impl` blocks at `std_messages`
Add the code related to richer error model support to a new
`richer_error` module. This improves readability and hopefully will
make it easier to add new features to `tonic-types` in the future.