Proposed solution to #1637 (fixes for lessc 1.5.1)

Fix some lessc issues exposed in #1637:

* Disable `clean-css` fallback for the deprecated `--yui-compress` argument as they are not compatible in their effects (see issue #1637) and simply ignore it.
* Print usage and stop execution when an invalid argument is provided (rather than ignore it). This may help developers to see faster that an argument is mispelled or deprecated.
* Change the exit status if the execution stops unsuccessfully. This may help the developer to see something went wrong during the build process, rather than experiencing bugs when using the CSS.