Make sure invalid path errors appear clearly in IOC console

When a start writing command is sent to the odin server with an invalid path set, it returns a 503 and logs

```
[Fri Sep 5 16:23:54 2025][E 250905 16:23:54 frame_processor_adapter:203] Error: Invalid path specified [/dls/i02_1/data/]
```

but ADOdin just prints

```
2025/09/05 16:23:54.527 RestParam[ODIN_HDF5_WRITE -> write]::basePut: Underlying RestAPI put failed
```

Make sure we print a useful error here. Things to check
- The response contains a useful error
- Errors from the FPs about invalid file paths are reported, even if the path exists in the control server (which could be on a different machine)

The first step is to reproduce the problem with fastcs-odin running in debug and check what the HTTP response is for
- sending a bad file path
- sending a start writing command with a bad file path

Dev environment:
- podman run --rm -it -v /dev/shm:/dev/shm --net=host ghcr.io/diamondlightsource/eiger-detector-runtime:latest
- fastcs-odin in devcontainer, launch Odin IOC

If we don't get nice responses, create an odin-data issue to fix it and pause this issue.

## Acceptance Criteria
- A clear error is logged when an invalid path is used