HgLoaderFromDisk supports corrupted revisions

Summary:
When a repository has corrupted revision, the revision and its descendants
are not loaded.

This commit only deals with missing filelogs and configures the exclusion
system.

Missing filelogs are recoverable errors that should be skipped or
saved as SkippedContent but being missing the SkippedContent cannot
be calculated. This point is left for future commits.

Reviewers: Reviewers

Subscribers: ardumont, Alphare, vlorentz

Differential Revision: https://forge.softwareheritage.org/D4688