txn feature request: more info on cas failure

A cas operation normally returns two results (a boolean result indicating whether the check operation succeeded or not and a server error).

However, a failure on cas in txn gives us a single error message. If we need to know the cause of the error, we must parse the error string which is quite unreliable.

For example, suppose that I don't have a write permission on key `key1`. The error response looks like the following:
```
HTTP/1.1 409 Conflict
{"Results":null,"Errors":[{"OpIndex":0,"What":"Permission denied"}]}
```

If a check operation fails, then the error response would look like this:
```
HTTP/1.1 409 Conflict
{"Results":null,"Errors":[{"OpIndex":0,"What":"failed to set key \"key1\", index is stale"}]}
```

The only way to distinguish it from other server errors is checking the error message itself and no one would like it.