Implement import recursion check

[mgreter]

Also adds import stack reports on errors.

Fixes #1801
https:/sass/sass-spec/pull/643/files

Error: An @import loop has been found:
           index.scss imports _alpha.scss
           _alpha.scss imports _beta.scss
           _beta.scss imports _alpha.scss
        from line 1 of index.scss
        from line 1 of _alpha.scss
        from line 1 of _beta.scss
>> @import 'alpha';