Ensure compliance with secondary TRAPI specifications

[tokebe]

Outside of the main TRAPI spec, there are secondary specifications and implementation guidance here which specify additional required implementations requirements (MUST log xyz, MUST return errors, etc.)

Currently, these haven't been explicitly included in Retriever's implementation, but we should conform to them where feasible.

[tokebe]

Additional items:

• bypass_cache behavior (subq dispatch + passing along to backends?)

[tokebe]

Addressed by #131, will track bypass_cache separately