Extract ChangeModifiedException out of PatchSetInserter to its own file

Coverity suggests that PatchSetInserter.ChangeModifiedException should be
a static class.  See CID 20102 on the coverity scan project [1].

On the review of change I6039cc43 where this class was originally added,
Dave Borowitz suggested moving it out to its own file. Do that instead of
making it a static class.

[1] https://scan.coverity.com/projects/4032

Change-Id: I37ee040a8426f3144ee2bcd9f3618958e263a740
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/PatchSetInserter.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/PatchSetInserter.java
index 98d99a3..baddd40 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/PatchSetInserter.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/PatchSetInserter.java
@@ -43,6 +43,7 @@
 import com.google.gerrit.server.notedb.ReviewerState;
 import com.google.gerrit.server.patch.PatchSetInfoFactory;
 import com.google.gerrit.server.project.ChangeControl;
+import com.google.gerrit.server.project.ChangeModifiedException;
 import com.google.gerrit.server.project.InvalidChangeOperationException;
 import com.google.gerrit.server.project.NoSuchChangeException;
 import com.google.gerrit.server.ssh.NoSshInfo;
@@ -380,12 +381,4 @@
     return changeMessage != null && changeMessage.getKey().getParentKey()
         .equals(patchSet.getId().getParentKey());
   }
-
-  public class ChangeModifiedException extends InvalidChangeOperationException {
-    private static final long serialVersionUID = 1L;
-
-    public ChangeModifiedException(String msg) {
-      super(msg);
-    }
-  }
 }
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/project/ChangeModifiedException.java b/gerrit-server/src/main/java/com/google/gerrit/server/project/ChangeModifiedException.java
new file mode 100644
index 0000000..f3ca8b6
--- /dev/null
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/project/ChangeModifiedException.java
@@ -0,0 +1,23 @@
+// Copyright (C) 2013 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.project;
+
+public class ChangeModifiedException extends InvalidChangeOperationException {
+  private static final long serialVersionUID = 1L;
+
+  public ChangeModifiedException(String msg) {
+    super(msg);
+  }
+}