* volume: reject needle blob writes to read-only volumes
WriteNeedleBlob appends the blob to .dat and only then calls nm.Put. On a
read-only volume the needle map is a SortedFileNeedleMap whose Put always
fails, so the append is never indexed and never rolled back.
Nothing upstream stops this: volume.check.disk picks its targets from the
master's cached topology, which goes stale the moment a volume server marks
a replica read-only itself — a failed data integrity check at load, or an
EIO quarantine. Each sync attempt then grows the .dat of a replica that is
supposed to be frozen by one unindexed needle, and reports it as "invalid
argument", the bare os.ErrInvalid the needle map returns.
Check IsReadOnly before touching .dat, same as the upload path does.
* volume: say which needle and volume failed to index
An index write that fails surfaced as a bare errno with no volume, no needle
and no file — "invalid argument" for a read-only needle map, or a plain
ENOSPC when .idx lives on its own filesystem via -dir.idx. Both were logged
at V(4), so by default the operator saw only the errno the client got back.