Issue 319 - CI Builds not treating test failures as build failures

This PR fixes #319. It was necessary to use slightly different settings for msbuild and xbuild. The Appveyor build now uses an argument that causes ContinueOnError to be set to "ErrorAndContinue" rather than "true". This results in the correct behavior under both travis-ci and appveyor.

To make it work, the travis-ci build now stops on the first test run failure. Since the build is divided into four jobs, this is not a great problem. The Appveyor build executes ten different test runs and reports failure if any of them fail.