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
2 files changed