HttpJson ServerSideStreaming error in EchoClient

For Java, we have a test case that tries to mimic an exception in the middle of the stream. The full response payload should be `"The rain in Spain stays mainly on the plain!"` with each word as it's own response. The `Cancelled` exception would come after the `Spain` response.
```
@Test
public void testHttpJson_serverError_receiveErrorAfterLastWordInStream() {
String content = "The rain in Spain";
Status cancelledStatus =
Status.newBuilder().setCode(StatusCode.Code.CANCELLED.ordinal()).build();
ServerStream responseStream =
httpjsonClient
.expandCallable()
.call(ExpandRequest.newBuilder().setContent(content).setError(cancelledStatus).build());
Iterator echoResponseIterator = responseStream.iterator();

assertThat(echoResponseIterator.next().getContent()).isEqualTo("The");
assertThat(echoResponseIterator.next().getContent()).isEqualTo("rain");
assertThat(echoResponseIterator.next().getContent()).isEqualTo("in");
assertThat(echoResponseIterator.next().getContent()).isEqualTo("Spain");
CancelledException cancelledException =
assertThrows(CancelledException.class, echoResponseIterator::next);
assertThat(cancelledException.getStatusCode().getCode()).isEqualTo(StatusCode.Code.CANCELLED);
}
```

Error:
```
...
Caused by: com.google.api.gax.httpjson.HttpJsonStatusRuntimeException: Exception in message delivery
at com.google.api.gax.httpjson.HttpJsonClientCallImpl.deliver(HttpJsonClientCallImpl.java:316)
... 54 more
Caused by: com.google.api.gax.httpjson.RestSerializationException: com.google.gson.stream.MalformedJsonException: Unterminated object at line 12 column 26 path $[3].severity
at com.google.api.gax.httpjson.ProtoMessageJsonStreamIterator.next(ProtoMessageJsonStreamIterator.java:131)
at com.google.api.gax.httpjson.HttpJsonClientCallImpl.consumeMessageFromStream(HttpJsonClientCallImpl.java:362)
at com.google.api.gax.httpjson.HttpJsonClientCallImpl.deliver(HttpJsonClientCallImpl.java:311)
... 54 more
Caused by: com.google.gson.stream.MalformedJsonException: Unterminated object at line 12 column 26 path $[3].severity
at com.google.gson.stream.JsonReader.syntaxError(JsonReader.java:1659)
at com.google.gson.stream.JsonReader.doPeek(JsonReader.java:500)
at com.google.gson.stream.JsonReader.peek(JsonReader.java:433)
at com.google.api.gax.httpjson.ProtoMessageJsonStreamIterator.next(ProtoMessageJsonStreamIterator.java:82)
... 56 more
```
which seems to stem from the response coming back as:
```
[{
"content": "The",
"severity": "UNNECESSARY"
},{
"content": "rain",
"severity": "UNNECESSARY"
},{
"content": "in",
"severity": "UNNECESSARY"
},{
"content": "Spain",
"severity": "UNNECESSARY"
}{"error":{"code":499,"message":"","details":[],"Body":"","Header":null,"Errors":null}}]
```

The exception above seems to be complaining that there is a missing `,` between the payload and the error message, which I believe is caused from:
EchoService impl: https://github.com/googleapis/gapic-showcase/blob/6d0f5b789ff1539195d8c135f8808681dd7c9f75/server/services/echo_service.go#L54
- Streaming constructing the array: https://github.com/googleapis/gapic-showcase/blob/6d0f5b789ff1539195d8c135f8808681dd7c9f75/util/genrest/resttools/server_streamer.go#L107
- Error response: https://github.com/googleapis/gapic-showcase/blob/6d0f5b789ff1539195d8c135f8808681dd7c9f75/util/genrest/resttools/error_response.go#L68

A few questions:
Would an exception that comes back in the middle of stream come back as a json response? Or would we have an expectation that the trailer message would have an exception with Status Code 499? I see from the local debugging that the status code currently comes back as a 200 (which I believe is due to https://cs.opensource.google/go/go/+/refs/tags/go1.17.1:src/net/http/server.go;l=120-126)

What should the correct behavior be?