Make MergeConflictException a ResourceConflictException

This means we don't have to unwrap this exception type when used from
a BatchUpdate, since BatchUpdate propagates RestApiExceptions.

Change-Id: I3b655308ec22913bee0b9114074d192360466893

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

@@ -0,0 +1,28 @@
+// Copyright (C) 2014 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.extensions.restapi;
+
+/**
+ * Indicates that a commit cannot be merged without conflicts.
+ * <p>
+ * Messages should be viewable by end users.
+ */
+public class MergeConflictException extends ResourceConflictException {
+  private static final long serialVersionUID = 1L;
+
+  public MergeConflictException(String msg) {
+    super(msg, null);
+  }
+}