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).
* 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 developers to see something went wrong during the build process, rather than experiencing bugs when using the CSS.