Support error message with backtrace

lib/sassc/error.rb

@@ -11,17 +11,19 @@ class UnsupportedValue < BaseError; end
   # it's important to provide filename and line number information.
   # This will be used in various error reports to users, including backtraces;
   class SyntaxError < BaseError
+    LINE_INFO_REGEX = /on line (\d+) of (.+)/
+
     def backtrace
       return nil if super.nil?
       sass_backtrace + super
     end
 
     # The backtrace of the error within Sass files.
     def sass_backtrace
-      line_info = message.split("\n")[1]
+      line_info = message.split("\n").find { |line| line.match(LINE_INFO_REGEX) }
       return [] unless line_info
 
-      _, line, filename = line_info.match(/on line (\d+) of (.+)/).to_a
+      _, line, filename = line_info.match(LINE_INFO_REGEX).to_a
       ["#{Pathname.getwd.join(filename)}:#{line}"]
     end
   end

test/error_test.rb

@@ -17,6 +17,21 @@ def test_first_backtrace_is_sass
         expected = "#{Pathname.getwd.join(filename)}:#{line}"
         assert_equal expected, err.backtrace.first
       end
+
+      begin
+        raise SassC::SyntaxError.new(<<-ERROR)
+Error: no mixin named border-radius
+
+       Backtrace:
+       \t#{filename}:#{line}
+        on line #{line} of #{filename}
+>>     @include border-radius(5px);
+   -------------^
+        ERROR
+      rescue SassC::SyntaxError => err
+        expected = "#{Pathname.getwd.join(filename)}:#{line}"
+        assert_equal expected, err.backtrace.first
+      end
     end
   end
 end