I too had this exact same error message!
My recommendation is to replace the "Run unit tests with MSTest" step. with an "Execute Windows batch command" step. This worked for me.
Command
del TestResults.trx
"C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\MSTest.exe" /testcontainer:Example.Tests\bin\Debug\Example.Tests.dll /resultsfile:TestResults.trx
Using this technique you may still use the "Publish MSTest test result report" step specifying...
Test report TRX file
TestResults.trx
Good luck!
Also, you can "simulate" the "Continue on failed tests" functionality with an "EXIT" call, seen below.
del TestResults.trx
"C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\MSTest.exe" /testcontainer:Example.Tests\bin\Debug\Example.Tests.dll /resultsfile:TestResults.trx /nologo
EXIT /B 0