Split `RuntimeError` and `RequestRejection` by protocol (#2517)

As outlined in the [Protocol Specific Errors] of the [Service Builder
Improvements RFC], `RuntimeError` should be split up into smaller,
protocol specific, errors which accurately model the failure cases of
each protocol.

The same goes for `RequestRejection`.

Closes #1703.

[Protocol Specific Errors]: https://github.com/awslabs/smithy-rs/blob/main/design/src/rfcs/rfc0020_service_builder.md#protocol-specific-errors
[Service Builder Improvements RFC]: https://github.com/awslabs/smithy-rs/blob/main/design/src/rfcs/rfc0020_service_builder.md