RER::DataSource::Transilien: report 403 errors more accurately
RER::DataSource::Transilien: report 403 errors more accurately

403 errors seem to happen for two reasons: either due to an exceeded call
quota, or due to passing a wrong station UIC code (and possibly other
situations).

Check whether a "Retry-After" HTTP header is present and report the 403 error
accordingly.

lcorbasson/rer-web,lcorbasson/rer-web,lcorbasson/rer-web,lcorbasson/rer-web