Discovered in pre-commit/pre-commit.com#113

Here's a simple test case:

$ tail -n999 scss/*
==> scss/main.scss <==
@import 'scss/variables';
==> scss/_variables.scss <==

Before:

$ ~/workspace/pre-commit.github.io/venv/bin/sassc --version
sassc 0.12.3 (sass/libsass 3.4.3)
$ ~/workspace/pre-commit.github.io/venv/bin/sassc scss/main.scss out/out.css
$ echo $?
0

After:

$ ~/workspace/pre-commit.github.io/venv/bin/sassc --version
sassc 0.13.0 (sass/libsass 3.4.4)
$ ~/workspace/pre-commit.github.io/venv/bin/sassc scss/main.scss out/out.css
sassc: error: Error: File to import not found or unreadable: scss/variables.
       Parent style sheet: /tmp/foo/scss/main.scss
        on line 1 of scss/main.scss
>> @import 'scss/variables';
   ^