Errors do not contain a backtrace

[lunelson]

I originally posted about this on node-sass; but it seems to be a bug in libsass so I'm reposting here. Certain errors are coming through without a backtrace, notably those thrown with the @errordirective itself. I made a repo that demonstrates the problem—simply clone and run npm i && npm t to see the comparison of @warn VS @error: the warning traces back through the import tree, the error does not.

WARNING: this is a warning
Backtrace:
        src/_warn-fn.scss:2, in function `warn`
        src/_partial.scss:10, in mixin `warn-error-test`
        src/test.scss:7

Error: this is an error
        on line 2 of src/_error-fn.scss
>>   @error '#{$str}';
   --^

https://github.com/lunelson/node-sass-error-trace

This only seems to happen with certain errors, and I thought I'd see if I could just post a PR to sass-spec that would show which ones fail; but I couldn't get it working on my system, and in any case the error testing there doesn't even deal with imports and backtraces (they are all flat, single-file), so that's probably why this has gone unnoticed.

node-sass       4.7.2   (Wrapper)       [JavaScript]
libsass         3.5.0.beta.2    (Sass Compiler) [C/C++]