Fix issue when error response is receieved for streaming output

Fixes the second half of #1006.  Now you'll get a proper error
message:

```
$ aws s3api get-object --bucket b --key k /tmp/outfile

A client error (InvalidRequest) occurred when calling the GetObject
operation: The object was stored using a form of Server Side Encryption.
The correct parameters must be provided to retrieve the object.

```

Closes #1006.

cc @kyleknap @danielgtaylor