Do not emit error code when stream is committed

If an error occurs when writing the body of a response, e.g.,
TimeoutException, previously S3Proxy would try to emit the AWS error
code.  Since this stream was already opened and closed, this failed
and Jetty emitted this warning:

Could not send response error 500: java.lang.IllegalStateException:
STREAM

References #303.

src/main/java/org/gaul/s3proxy/S3ProxyHandler.java

@@ -2856,6 +2856,11 @@ protected final void sendSimpleErrorResponse(
             Map<String, String> elements) throws IOException {
         logger.debug("sendSimpleErrorResponse: {} {}", code, elements);
 
+        if (response.isCommitted()) {
+            // Another handler already opened and closed the writer.
+            return;
+        }
+
         response.setStatus(code.getHttpStatusCode());
 
         if (request.getMethod().equals("HEAD")) {