Change wallet method disabled error text

Not strictly backwards compatible because the error is not new in this release.

src/wallet/rpcwallet.cpp

@@ -60,8 +60,13 @@ bool EnsureWalletIsAvailable(CWallet * const pwallet, bool avoidException)
     if (pwallet) return true;
     if (avoidException) return false;
     if (::vpwallets.empty()) {
-        // Wallet RPC methods are disabled if no wallets are loaded.
+        // Note: It isn't currently possible to trigger this error because
-        throw JSONRPCError(RPC_METHOD_NOT_FOUND, "Method not found (disabled)");
+        // wallet RPC methods aren't registered unless a wallet is loaded. But
+        // this error is being kept as a precaution, because it's possible in
+        // the future that wallet RPC methods might get or remain registered
+        // when no wallets are loaded.
+        throw JSONRPCError(
+            RPC_METHOD_NOT_FOUND, "Method not found (wallet method is disabled because no wallet is loaded)");
     }
     throw JSONRPCError(RPC_WALLET_NOT_SPECIFIED,
         "Wallet file not specified (must request wallet RPC through /wallet/<filename> uri-path).");