title: Custom visual studio extension error: "the extension manifest is invalid"
body: <p>I am trying to edit a small and simple visual studio extension project. Everything builds, but when I double click on the vsix file to install, I get a popup with the message "the extension manifest is invalid". What could cause this/where can I start looking?</p>
  
  <p>I am developing in and targeting Visual Studio 2012.</p>