on_build_error should honor cleanup setting

[analog-cbarber]

trafficstars

On a build error the temp directory is removed regardless of whether the cleanup config option is enabled. This hampers debugging.

Should it be something like the following?

    def on_build_error(self, error):
        if self.temp_dir and self.config.get("cleanup"):
            shutil.rmtree(str(self.temp_dir))