rpc: Improve getblock / getblockstats error when only header is available.

This improves the error message of the getblock and getblockstats rpc and prevents calls to
ReadRawBlockFromDisk(), which are unnecessary if we know
from the header nStatus field that the block is not available.

test/functional/feature_assumeutxo.py

@@ -313,7 +313,7 @@ class AssumeutxoTest(BitcoinTestFramework):
         self.connect_nodes(snapshot_node.index, miner.index)
         self.sync_blocks(nodes=(miner, snapshot_node))
         # Check the base snapshot block was stored and ensure node signals full-node service support
-        self.wait_until(lambda: not try_rpc(-1, "Block not found", snapshot_node.getblock, snapshot_block_hash))
+        self.wait_until(lambda: not try_rpc(-1, "Block not available (not fully downloaded)", snapshot_node.getblock, snapshot_block_hash))
         self.wait_until(lambda: 'NETWORK' in snapshot_node.getnetworkinfo()['localservicesnames'])
 
         # Now that the snapshot_node is synced, verify the ibd_node can sync from it
@@ -485,7 +485,7 @@ class AssumeutxoTest(BitcoinTestFramework):
         # find coinbase output at snapshot height on node0 and scan for it on node1,
         # where the block is not available, but the snapshot was loaded successfully
         coinbase_tx = n0.getblock(snapshot_hash, verbosity=2)['tx'][0]
-        assert_raises_rpc_error(-1, "Block not found on disk", n1.getblock, snapshot_hash)
+        assert_raises_rpc_error(-1, "Block not available (not fully downloaded)", n1.getblock, snapshot_hash)
         coinbase_output_descriptor = coinbase_tx['vout'][0]['scriptPubKey']['desc']
         scan_result = n1.scantxoutset('start', [coinbase_output_descriptor])
         assert_equal(scan_result['success'], True)
@@ -557,7 +557,7 @@ class AssumeutxoTest(BitcoinTestFramework):
         self.log.info("Submit a spending transaction for a snapshot chainstate coin to the mempool")
         # spend the coinbase output of the first block that is not available on node1
         spend_coin_blockhash = n1.getblockhash(START_HEIGHT + 1)
-        assert_raises_rpc_error(-1, "Block not found on disk", n1.getblock, spend_coin_blockhash)
+        assert_raises_rpc_error(-1, "Block not available (not fully downloaded)", n1.getblock, spend_coin_blockhash)
         prev_tx = n0.getblock(spend_coin_blockhash, 3)['tx'][0]
         prevout = {"txid": prev_tx['txid'], "vout": 0, "scriptPubKey": prev_tx['vout'][0]['scriptPubKey']['hex']}
         privkey = n0.get_deterministic_priv_key().key