Handle SetCustomMiningJobError in translator and JD mining upstream (#142)

[ANAMASGARD]

Summary

• Translator upstream: Replace unimplemented!() in handle_set_custom_mining_job_error with structured error logging and Ok(SendTo::None(None)), preventing process panic when the pool rejects token-update SetCustomMiningJob requests.
• JD mining upstream: On SetCustomMiningJobError, log the rejection reason, clear the pending request_id -> template_id mapping via take_template_id, and set IS_CUSTOM_JOB_SET so the job declarator is not blocked for up to 120s.
• Maintenance: Comment out SetCustomMiningJobError in the translator parse_incoming todo!() arm (consistent with SubmitSharesError); fix success-handler log typo (SetupConnectionSuccess → SetCustomMiningJobSuccess).

Closes #142

Changes

File	What changed
src/translator/upstream/upstream.rs	Error handler + parse_incoming cleanup + unit test
src/jd_client/mining_upstream/upstream.rs	Error handler with mapping cleanup + log typo fix + unit test

Test plan

• cargo build
• cargo test set_custom_mining_job_error
• cargo test (81 passed, 2 ignored)

What the tests prove

Test	Assertion
translator::upstream::upstream::tests::set_custom_mining_job_error_does_not_panic	Handler returns Ok(SendTo::None(None)) without panicking
jd_client::mining_upstream::upstream::tests::set_custom_mining_job_error_clears_request_mapping	request_id mapping removed; IS_CUSTOM_JOB_SET == true