GET /tx/:txid/outspend/:vout returns 200 status for transactions which do not exist #266

Problem

The response of GET /tx/:txid/outspend/:vout for a txid which does not exist is 200 with a payload

{
    "spent": false
}

which is identical to valid utxos which have not been spent.

for example:

https://blockstream.info/api/tx/b779bc24f17f0cea8dc556d4d1301642986517bfbbc631eab8a6454fca1202bb/outspend/0

returns 200 with the payload above but the transaction b779bc24f17f0cea8dc556d4d1301642986517bfbbc631eab8a6454fca1202bb does not exist.

That contradicts with the GET /tx/:txid endpoint which returns a 404 for non existent txids. Following the example above https://blockstream.info/api/tx/b779bc24f17f0cea8dc556d4d1301642986517bfbbc631eab8a6454fca1202bb returns 404:

Transaction not found

The response of GET /tx/:txid/outspend/:vout is non consistent when querying inexsistent utxos.

Expected behavior

I would expect GET /tx/:txid/outspend/:vout to return a 404 for utxos which do not exist or have been dropped.