Misleading error message: --utxos-limit failures report "Too many history entries"

[EddieHouston]

Summary

ErrorKind::TooPopular has the display string "Too many history entries" (src/errors.rs#L22-L25), but it is raised from two unrelated checks:

1. Electrum history queries — when results exceed --electrum-txs-limit (src/electrum/server.rs#L1011, #L414). The message is accurate here.
2. UTXO set computation — when the UTXO set exceeds --utxos-limit during history replay (src/new_index/schema.rs#L873-L876). The message is wrong here: the failing condition is utxos.len() > limit and has nothing to do with history entry count.

Both limits default to 500, but they measure different quantities and are configured independently. On a deployment where --electrum-txs-limit has been raised but --utxos-limit is left at its default, an address with more than 500 UTXOs serves its full history without complaint while GET /address/:addr/utxo and blockchain.scripthash.listunspent fail with "Too many history entries" — a reason the server just demonstrated is not the problem.

The REST layer returns the error string verbatim as an HTTP 400 body (src/rest.rs#L543), so the misleading message is customer-visible and sends anyone debugging toward the wrong limit.

Observed behavior (signet address with 523 history entries, ~522 UTXOs)

Request	Result
GET /address/{addr}	200 OK
GET /address/{addr}/txs	200 OK
blockchain.scripthash.get_history	200 OK — 523 entries
blockchain.scripthash.get_balance	200 OK
GET /address/{addr}/utxo	400 — "Too many history entries"
blockchain.scripthash.listunspent	Error — "Too many history entries"

A related source of confusion: the UTXO check aborts when the set exceeds the limit at any point during replay (deliberately — the server must still materialize the peak-size map), so an address whose current balance is zero can also be refused with "Too many history entries". With a message naming the actual condition, that behavior would at least be interpretable.

Suggested fix

Add a distinct error kind and raise it from utxo_delta, leaving TooPopular for the Electrum history checks:

TooManyUtxos {
    description("Too many unspent outputs")
    display("Too many unspent outputs")
}