error: add CensoredNodeError, will be thrown when content deliberately erased This change introduces the error plus a corresponding catch in dispatch, to provide localized error messages. The verb "censor" is used in this commit and all following to refer to erasing the content of a revlog revision (filelog, for now) without recalculating node IDs, leaving that revision invalid. Further work must be done to safely share such revision data with compliant clients. I find the analogy to censorship straightforward; for less politically charged options, consider "erase", "excise", "expunge", or "blackhole".