title: Using XmlMassUpdate to update all appSettings
body: <p>I want to use the XmlMassUpdate task in MSBuild to update appSettings in all app.configs. The problem I have is that some of the app.configs don't have an appSettings element, and I can't get XmlMassUpdate to skip those.</p>
  
  <p>Here's what I've got so far</p>
  
  <pre><code>&lt;ProjectExtensions&gt;
    &lt;appSettings xmlns:xmu="urn:msbuildcommunitytasks-xmlmassupdate"&gt;
      &lt;add xmu:key="key" key="SettingName" value="newSetting" xmu:action="update" /&gt;
    &lt;/appSettings&gt;
  &lt;/ProjectExtensions&gt;
  &lt;Target Name="Change"&gt;
     &lt;ItemGroup&gt;
       &lt;AppConfigFiles Include="$(SourceRoot)\**\App.config" /&gt;
     &lt;/ItemGroup&gt;
     &lt;XmlMassUpdate 
        ContentFile="$(SourceRoot)\%(AppConfigFiles.RecursiveDir)App.config"
        ContentRoot="/configuration/appSettings"
        NamespaceDefinitions="msb=http://schemas.microsoft.com/developer/msbuild/2003"
        SubstitutionsFile="$(MSBuildProjectDirectory)\SetConfig.proj"
        SubstitutionsRoot="/msb:Project/msb:ProjectExtensions/msb:appSettings" /&gt;
  &lt;/Target&gt;
  </code></pre>
  
  <p>It fails on one of the App.configs saying Unable to locate '/configuration/appSettings'</p>