`core`: Feign Implementation Details exposed to clients

[bstopp]

The use of the Feign library/framework is an implementation detail and shouldn't concern the consumers of the library.

However, every API call throws runtime exceptions all of which extend Feign's. That they are Runtime does abstract away many concerns. However, the APIs should throw declared exceptions (Runtime or not) so that consumers can properly handle error states, without relying on inspecting the underlying error details.