loader: Fix no longer existing hg node filtering

When a mercurial origin is loaded, SWH stores the mapping between the original hg revision ids and SWH revision ids as extids. This mapping is then reused in subsequent loads to determine which new revisions should be archived.

However it exist cases where previously seen hg revisions were removed from the mercurial repository between two loads by SWH.

Previously the code filtering such no longer existing revisions was not catching the right exception and thus an error was raised when attempting to reload such mercurial origins.

Related to #4583

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