storage_sync_prepare_push returns 13 with nothing on stdout or stderr

[fujibee]

storage_sync_prepare_push can fail with exit 13 and print nothing at all — not
on stdout, not on stderr. The caller is left with a number and no sentence.

Measured

Found while building a control for #829. A jq that emits CRLF poisons the
$( )-captured prepare fields, so the driver refuses — correctly. What it does
not do is say so:

rc=13
--- stderr ---
(empty)
--- stdout ---
(empty)

Both streams captured to files, both zero bytes.

Why it matters here specifically

13 is not a rare path. grep -c 'return 13' in
scripts/drivers/storage/sqlite-sync.sh finds it used for several distinct
conditions — a failed storage_init, a prepared/pending mismatch, a schema
problem — and they are indistinguishable from outside. An operator, or a test
author, sees one number for several causes and no text for any of them.

This is what made the #829 control hard to write: the control was failing, the
driver was refusing for a reason the test had created, and there was nothing to
read. The time went into bisecting a silence.

The shape

Same family as #802: a failure became a value, and nothing was recorded. The
difference is that here the value is returned rather than swallowed, so the
call does stop — what is missing is only the sentence. That makes it cheaper to
fix than #802's cases and no less worth fixing.

Suggested direction

Each return 13 site says which condition it is, on stderr, before returning.
The message does not need to be long; it needs to name the check. Two of them
already have text nearby in comments that would serve as the sentence.

Not proposed: changing the exit code, or splitting 13 into several. The callers
read the number and the number is part of the contract; this is about the text
beside it.