commit-graph: clarify missing-chunk error messages
When a required commit-graph chunk cannot be loaded, we leave its entry
in the struct NULL, and then later complain that it is missing. But
that's just one reason we might not have loaded it, as we also do some
data quality checks.
Let's switch these messages to say "missing or corrupted", which is
exactly what the midx code says for the same cases. Likewise, we'll use
the same phrasing and capitalization as those for consistency. And while
we're here, we can mark them for translation (just like the midx ones).
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>