rpc, cli: improve error message on multiwallet mode

The primary objective is to provide users with clearer
and more informative error messages when encountering
the RPC_WALLET_NOT_SPECIFIED error, which occurs when
multiple wallets are loadad.

This commit also rectifies the error message consistency
by bringing the error message in line with the definition
established in protocol.h ("error when there are multiple
wallets loaded").

test/functional/wallet_multiwallet.py

@@ -229,7 +229,7 @@ class MultiWalletTest(BitcoinTestFramework):
         assert_raises_rpc_error(-18, "Requested wallet does not exist or is not loaded", wallet_bad.getwalletinfo)
 
         # accessing wallet RPC without using wallet endpoint fails
-        assert_raises_rpc_error(-19, "Wallet file not specified", node.getwalletinfo)
+        assert_raises_rpc_error(-19, "Multiple wallets are loaded. Please select which wallet", node.getwalletinfo)
 
         w1, w2, w3, w4, *_ = wallets
         self.generatetoaddress(node, nblocks=COINBASE_MATURITY + 1, address=w1.getnewaddress(), sync_fun=self.no_op)
@@ -275,7 +275,7 @@ class MultiWalletTest(BitcoinTestFramework):
         loadwallet_name = node.loadwallet(wallet_names[1])
         assert_equal(loadwallet_name['name'], wallet_names[1])
         assert_equal(node.listwallets(), wallet_names[0:2])
-        assert_raises_rpc_error(-19, "Wallet file not specified", node.getwalletinfo)
+        assert_raises_rpc_error(-19, "Multiple wallets are loaded. Please select which wallet", node.getwalletinfo)
         w2 = node.get_wallet_rpc(wallet_names[1])
         w2.getwalletinfo()