Improve exception message when shard has a partial commit (segments_N file) due to prior disk full #11539
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ES checks the latest segments_N file to see if it was written by Lucene 3.x and upgrade if so, but if that file is truncated (e.g. 0 bytes) due prior disk full, we now give a confusing "failed to upgrade 3x segments" exception when you likely don't have 3.x segments_N nor segments.
This change just breaks apart the exception handling so we do a better job separating "I could not read the latest segments_N file" from "I did read it and it was ancient (Lucene 3.x) and then when I tried to upgrade it, bad things happened".
Closed #11249