bitcoind: make wallet-load sidecars retry instead of exiting

[openoms]

Wiki note: https://github.com/blinkbitcoin/blink-wip/wiki/openoms----2026-05-18-Bitcoind-Onchain-Wallet-Loading

Follow-up for the bitcoind chart wallet-loading script: retry/back off in place on transient RPC or loadwallet failures instead of exiting immediately.