Return Bad Request on bad reviewers to PostReview

Now that posting a review can be used to add reviewers, it's possible
for the entire post to fail due to bad input (e.g., an unresolvable
account, an oversized group). This information is already returned in
the JSON response body, but we should use a status code other than
200 OK when this happens so the client doesn't erroneously assume that
the review was successfully posted.

Change-Id: I763b5c73878b7c70a95953759b371a182c8a8dad

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/restapi/Response.java

@@ -52,6 +52,11 @@ public abstract class Response<T> {
     return new Redirect(location);
   }
 
+  /** Arbitrary status code with wrapped result. */
+  public static <T> Response<T> withStatusCode(int statusCode, T value) {
+    return new Impl<>(statusCode, value);
+  }
+
   @SuppressWarnings({"unchecked", "rawtypes"})
   public static <T> T unwrap(T obj) {
     while (obj instanceof Response) {