Refactor old code to properly raise Exceptions instead of returning error dictionaries

See:
https://github.com/fusion44/blitz_api/blob/72862f8d6a25d0951097e6e3db79e4d0c10cca3f/app/bitcoind/utils.py#L69

Example:
```python
except aiohttp.client_exceptions.ClientConnectionError as e:
return {
"error": f"Aiohttp client connection error: {str(e)}",
"status": status.HTTP_503_SERVICE_UNAVAILABLE,
}
```

utils.py - redis_get():
https://github.com/fusion44/blitz_api/blob/72862f8d6a25d0951097e6e3db79e4d0c10cca3f/app/api/utils.py#L57

This function returns an empty string when a key is not found in Redis. This is not good. An empty string could be a valid value for a key. The caller can get confused whether the result is an error or just an empty string.

Start with the linked exception-handling code in app/bitcoind/utils.py around line 69 and app/api/utils.py around line 57, then inspect their callers. Trace how connection failures and missing Redis keys are currently handled; done means the affected paths consistently distinguish exceptions from valid return values without relying on error dictionaries or an empty string.

Written by the indexing model from the issue text.