Make @error fatal. Resolves #967.

[asottile]

Manual testing as requested!

$ ./bin/sassc test.scss 
Error: hai
        on line 1 of test.scss
>> a { @error 'hai'; }
   ----^
$ echo $?
1

[coveralls]

Coverage increased (+0.01%) to 80.17% when pulling 05dc97d on asottile:error_fatal into bb10765 on sass:master.

[xzyfer]

Am I correct in remembering that error give us the backtrace trace that you've removed?

[asottile]

ah yeah I've made an unused variable. I'll remove that. You can see the sample output in the description of the PR

[asottile]

fixed!

[coveralls]

Coverage increased (+0.03%) to 80.19% when pulling 21cc378 on asottile:error_fatal into bb10765 on sass:master.

[xzyfer]

Yeah cool just wanted to make sure backtraces were maintained. It's hard to tell from the example since there's not nesting to unwind.

I have confirmed this is the case - https://github.com/sass/libsass/blob/master/error_handling.cpp#L14-L21

Great work! Thanks!