If a file header is corrupted then the exception may be reported as a
bad index format version rather than a checksum mismatch. This commit
adjusts the docs to cover this case.
These docs talk about files whose contents are unexpected, but we should
also mention that files which are completely missing are also going to
be due to infrastructural problems.
Adds some docs giving more detailed background about what data
corruption really means and some suggestions about how to narrow down
the root cause.
Co-authored-by: Henning Andersen <33268011+henningandersen@users.noreply.github.com>
