Lets the error propagate to libsass.

This goes against the concept of fast aborting,
but this way downstream user will not get a blunt
exception, but receive a well-formatted JSON
error.

Note: this exception from importer callback's
post-process handler cannot be captured by try-
catch.

The edit in binding.cpp is also required, because
those conditions will never get satisfied anyway.
Removed due to aforementioned reasons.