The repo last_synced_revisions value is not validated

[eemeli]

Pontoon keeps track of the last synced revision identifier, and uses this when determining what files have changed. However, if the repo has changed and that commit/changeset id is no longer valid, the current fallback behaviour is to swallow the error and to return an empty list. This means that any changed or removed files won't be detected and updated until they next change.

On error, we should probably default to returning all the files, which is what's done if the revision id is missing.