Error handling in RedisIO writes

The writes in RedisIO currently don't check the MULTI response for errors, so issues might remain unnoticed. The typical pattern should be something like:
```

Response> resp = pipeline.exec();
pipeline.close(); // does sync internally
resp.get();
// this may throw

```

See discussion here https://github.com/apache/beam/pull/15858#discussion_r765719311

Imported from Jira [BEAM-13458](https://issues.apache.org/jira/browse/BEAM-13458). Original Jira may contain additional context.
Reported by: mosche.