fts_read: Handle error from a NULL return better.

[bdrewery]

Summary

This is addressing cases such as fts_read(3) encountering an [EIO]
from fchdir(2) when FTS_NOCHDIR is not set. That would otherwise be
seen as a successful traversal in some of these cases while silently
discarding expected work.

As noted in r264201, fts_read() does not set errno to 0 on a successful
EOF so it needs to be set before calling it. Otherwise we might see
a random error from one of the iterations.

gzip is ignoring most errors and could be improved separately.

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