Handle nil as a sass_filename. Closes #1883.

Author: chriseppstein

cli/lib/compass/sass_compiler.rb

@@ -113,10 +113,11 @@ def when_deleting_sourcemap(filename)
   def when_compilation_error(error, sass_file, css_file, sourcemap_file)
     @error_count += 1
     if error.respond_to?(:sass_filename)
-      if relativize(error.sass_filename) == relativize(sass_file)
-        logger.record :error, "#{relativize(error.sass_filename)} (Line #{error.sass_line}: #{error.message})"
+      error_filename = error.sass_filename || sass_file
+      if relativize(error_filename) == relativize(sass_file)
+        logger.record :error, "#{relativize(sass_file)} (Line #{error.sass_line}: #{error.message})"
       else
-        logger.record :error, "#{relativize(sass_file)} (Line #{error.sass_line} of #{relativize(error.sass_filename)}: #{error.message})"
+        logger.record :error, "#{relativize(sass_file)} (Line #{error.sass_line} of #{relativize(error_filename)}: #{error.message})"
       end
     else
       logger.record :error, "#{relativize(sass_file)} (#{error.backtrace.first}: #{error.message})"