chore: debug relay responses

[ltitanb]

before we were using Display which was creating some annoying gaps in the stdout logs

Before

2025-01-21T16:58:58.807625Z  WARN cb_pbs::mev_boost::submit_block::tests: failed to get payload (this might be ok if other relays have it) err=relay response error. Code: 400, err: {"code":400,"message":"no execution payload for this request - block was never seen by this relay"}
{"code":400,"message":"no execution payload for this request"}

2025-01-21T16:58:58.807733Z  WARN cb_pbs::mev_boost::submit_block::tests: failed to get payload (this might be ok if other relays have it) err=relay response error. Code: 400, err: {"code":400,"message":"no execution payload for this request - block was never seen by this relay"}
{"code":400,"message":"no execution payload for this request"}

After

2025-01-21T17:01:18.019866Z  WARN cb_pbs::mev_boost::submit_block::tests: failed to get payload (this might be ok if other relays have it) err=relay response error. Code: 400, err: "{\"code\":400,\"message\":\"no execution payload for this request - block was never seen by this relay\"}\n{\"code\":400,\"message\":\"no execution payload for this request\"}\n"
2025-01-21T17:01:18.019920Z  WARN cb_pbs::mev_boost::submit_block::tests: failed to get payload (this might be ok if other relays have it) err=relay response error. Code: 400, err: "{\"code\":400,\"message\":\"no execution payload for this request - block was never seen by this relay\"}\n{\"code\":400,\"message\":\"no execution payload for this request\"}\n"