Add display impl for errors variants in apps

[bit-aloo]

I don't really like seeing phantom types in error. Its an internal detail, and shouldn't be exposed in logs. This PR adds impl display trait for error variants across apps.

[par1ram]

Reviewed the four error hierarchies and the logging call-site conversions; I found no blocking correctness issues. On the current merge with main, the 33 existing jd-client/translator tests pass, the pool/JDS crates build and test successfully, and four temporary contract tests confirmed that representative Display outputs preserve owner/action/cause context without leaking PhantomData.

Non-blocking: please consider retaining downstream_id as a dedicated tracing field in the Action::Disconnect branches even though it is included in %e; this keeps operational filtering intact. A small table-driven Display test would also make this new logging contract explicit, especially since pool and JDS currently have no unit tests.