The code already only cares about the _errorMessage when _showError is
true. By using a falsy value for when there is no error, there is less
state to keep in sync, and it will be easier to make this a public
property when we later split the loading code out of the gr-diff widget.
Bug: Issue 9623
Change-Id: Ib9316aabdf2a4c3e5976885afbfc1d61a283d054