Fail gracefully if the revision decoding process fails

Currently, we assume that the revision data (except its message) are always utf8-encoded. We would like to have the ability to catch any and all decoding failures within the converter, and to provide another manner of accessing the content we were not able to decode (downloading the raw data?), in the same fashion as the revision message is being handled.

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Migrated from T435 (view on Phabricator)