mimetype indexer: work around mimetype detection problem to unstuck indexing workers

As my priority is for now not the indexer, i don't have time to find the proper way to fix the mimetype indexer (more information in the parent task #861 (closed)). So taking a shortcut to unstuck the indexers to avoid having them stuck in a rescheduling loop hell.

The idea is to trap the error raised. Log in error the hash and continue indexing the rest. A pattern like Detecting mimetype error for id <hash> will help. This way, we can compulse a list of sha1 to re-index later on when the fix is found and deployed.

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Migrated from T862 (view on Phabricator)