Expose remote error fields to client

When I attempt an `INSERT` that violates a primary key, the message seen by the client is a generic `WARNING` about a bad result from the worker node rather than the more meaningful message originally produced by the remote error:

``` sql
INSERT INTO members VALUES (1, 2, 3);

INSERT INTO members VALUES (1, 2, 3);

# WARNING: Bad result from shard1.demo:5432
# DETAIL: Remote message: duplicate key value violates unique constraint "members_id_key_10007"
# WARNING: Bad result from shard8.demo:5432
# DETAIL: Remote message: duplicate key value violates unique constraint "members_id_key_10007"
# ERROR: could not modify any active placements
```

This is a problem because:
- many languages don't capture the detail, just the error message
- the error message is cryptic in the extreme