This method uses Sass error value
as a vehicle to report the error message,
but returns NULL to indicate constructor
failure.

This is used to tell successful creation
of the Sass error object from the constructor
failure, where no valid Sass object can be
created.

::construct() methods may return NULL and
indicate an error in the returned error value.

In the unlikely event libsass gives us the
value we don't understand use V8 exception
to report failure and exit before calling
a user-defined custom function.

XXX We do not have currently any way
    to test this (that would require mocking
    libsass).

Fix crash in List::SetValue() etc. when trying
to add a bare JavaScript object.

Check for bare objects in lists and maps in test.

Don't use C++ exceptions in the binding code.