Throw error code when build fails

When using `typescript-loader` with webpack, failed compilations cause an exit code of `0` to be returned, even when the `--bail` flag is set on webpack (which is soon going to be switched to the default behavior: https://github.com/webpack/webpack/issues/708).

This breaks build tools like `make` and Docker, which use exit codes to determine whether a process encountered an error or not.