Log warning when statusCode != 200 is returned

It also adds a test when a non-existent domain was requested.

Had to put `npm test` to use `grunt --force` to test warnings that could be logged by grunt-curl. I've executed the tests with grunt 0.4.1 and 0.3.17.

From the grunt docs:

grunt.fail.warn('Something went wrong.'); logs a warning in bright yellow, exiting grunt with exit code 1, unless --force was specified.