Better error return

[sveetch]

Reported error from pure sass syntax cause is not a problem, output is relevant and sufficient enough.

BUT when libsass cause a segmentation fault/core dump, there is no output.

I don't think we can manage something to have more output than just the core dump message, but at least the main compiled file causing it should be displayed.

Actually compiled file name is displayed only when the compile is finished. It should be usefull to know what file to search for.