[Bug]: Clarify streaming errors in REST transport binding

### What happened?

Currently there's no information on how to return errors for streaming requests in REST transport binding.
For JSONRPC which also uses SSE for streaming we return a data line with jsonrpc error struct in it.
```
data: {"jsonrpc": "2.0", "id": 1, "result": { /* Task | Message | TaskArtifactUpdateEvent | TaskStatusUpdateEvent */ }}

data: {"jsonrpc": "2.0", "id": 1, "error": { /* details */ }}
```

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