add a grunt.warn message if any tests fail

Currently if any of the mocha tests called from within grunt simplemocha fail, it emits an error like:

    Warning: Task "simplemocha:server" failed. Use --force to continue.
    Error: Task "simplemocha:server" failed.
        at Task.<anonymous (.../grunt/lib/util/task.js:197:15)
        at null._onTimeout (.../grunt/lib/util/task.js:225:33)
        at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)
    Aborted due to warnings.

By adding the call to grunt.warn this turns into the much nicer:

    Warning: 1/6 tests failed. Use --force to continue.
    Aborted due to warnings.