Call completeStream only once on failure.

Fix #13470 by calling completeStream only once even when there is a failure in the channel callback.

* Call completeStream only once on failure.
* refactor success and failure into single method.
* Improved EventSourceServlet

Author: gregw

jetty-core/jetty-client/src/test/java/org/eclipse/jetty/client/HttpClientFailureTest.java

@@ -175,7 +175,7 @@ public void testPendingRequestContentThenTotalTimeout() throws Exception
                     completed.incrementAndGet();
                     assertThat(result.getRequestFailure(), notNullValue());
                     assertThat(result.getResponseFailure(), nullValue());
-                    assertThat(result.getResponse().getStatus(), is(HttpStatus.OK_200));
+                    assertThat(result.getResponse().getStatus(), is(HttpStatus.INTERNAL_SERVER_ERROR_500));
                     resultLatch.countDown();
                 });