dist/licenses: add and use diff-filter

Mitigate against new behavior introduced by https://github.com/RIOT-OS/RIOT/pull/2037 which leads to unintended effects like https://travis-ci.org/RIOT-OS/RIOT/builds/41797950 in https://github.com/RIOT-OS/RIOT/pull/1803.
Also, add a `--error-exit-code` option to be able prevent tests from failing that can be ignored anyways.