Check for and disallow pushing of invalid refs/meta/config

If the project.config or groups files are somehow invalid on
the refs/meta/config branch, or would be made invalid due to
a bad code review being submitted to this branch, reject the
user's attempt to push.

Bug: issue 1002
Change-Id: I9fafd651d875f50f501bd2a113441c2a47b3c0fa

gerrit-server/src/main/java/com/google/gerrit/server/git/ValidationError.java

@@ -0,0 +1,41 @@
+// Copyright (C) 2011 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.server.git;
+
+/** Indicates a problem with Git based data. */
+public class ValidationError {
+  private final String message;
+
+  public ValidationError(String file, String message) {
+    this(file + ": " + message);
+  }
+
+  public ValidationError(String file, int line, String message) {
+    this(file + ":" + line + ": " + message);
+  }
+
+  public ValidationError(String message) {
+    this.message = message;
+  }
+
+  public String getMessage() {
+    return message;
+  }
+
+  @Override
+  public String toString() {
+    return "ValidationError[" + message + "]";
+  }
+}