Categorize scss-lint syntax errors as Syntax subtype

Previously, all errors/warnings reported by `scss-lint` were considered
style errors in Syntastic.

Commit https://github.com/causes/scss-lint/commit/25ec426999230d74399fb133cdb483ca5ac3eb68
(which will be released in version 0.30.0 of `scss-lint`) includes the
prefix "Syntax Error" at the beginning of all syntax errors, so it can
be easily detected whether an error is a style or syntax error.

There's no need to add a version check as we're fine with older versions
of `scss-lint` continuing to report syntax errors as style errors.

Closes #1189.