The "Message" task was not found

I restarted my computer and the error disappeared :)

Updated:

Based on the below comments (Thanks to @Thomas & @Vikram), it seems that restarting Visual Studio also fixes the issue.

I had the same problem in VS 2013 and fixed it by cleaning the solution.

Just right click on the offending project and select "Clean". It's a bit faster than restarting VS. I still don't know what is causing this issue for me but that fixes it so that I can build the project.