rpc: Include assumeutxo as a failure reason of rescanblockchain

test/functional/wallet_assumeutxo.py

@@ -176,6 +176,10 @@ class AssumeutxoTest(BitcoinTestFramework):
         assert_equal(result[0]['error']['code'], -1)
         assert_equal(result[0]['error']['message'], expected_error_message)
 
+        self.log.info("Test that rescanning blocks from before the snapshot fails when blocks are not available from the background sync yet")
+        w1 = n1.get_wallet_rpc(wallet_name)
+        assert_raises_rpc_error(-1, "Failed to rescan unavailable blocks likely due to an in-progress assumeutxo background sync. Check logs or getchainstates RPC for assumeutxo background sync progress and try again later.", w1.rescanblockchain, 100)
+
         PAUSE_HEIGHT = FINAL_HEIGHT - 40
 
         self.log.info("Restarting node to stop at height %d", PAUSE_HEIGHT)