global: replace most uses of RevlogError with StorageError (API)
When catching errors in storage, we should be catching
StorageError instead of RevlogError. When throwing errors related
to storage, we shouldn't be using RevlogError unless we know
the error stemmed from revlogs. And we only reliably know that
if we're in revlog.py or are inheriting from a type defined in
revlog.py.
Differential Revision: https://phab.mercurial-scm.org/D4655
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