launchpad: Manage unhandled exceptions when listing

Prior to this commit, the listing could fail when either reading a page or the page of
results (lauchpad api raises RestfulError). This now retries when those kind of
exceptions happen. If the error persists (after multiple tryouts and exponential
backoff), the listing continues nonetheless (with warning logs).

Note that if the page ends up being empty, it's no longer accounted for.

This actually allows the listing to finish in case of issues.

Related to T3945