Running sass tasks is aborted due to warnings caused by Susy 3.0+ dropping all mixins.

Relevant issue on the Susy repo and a blog post.

That’s right, Susy3 has no mixins. When we wrote the mixins for Susy1 and Susy2, floats were (practically) the only option, and required a fair amount of hacking help. Mixins provided those shortcuts, at the expense of user flexibility. But mixins also created a black-box of CSS output, which made it more difficult for users to understand the code they were writing.

Error output:

Running "sass:main" (sass) task
Error: no mixin named container
>>         on line 11 of .dev/sass/components/_menus.scss
>>         from line 94 of .dev/sass/style.scss
>> >> 	@include container($container-size);

Proposed solution:
Downgrade Susy to version 2.2.14