wallet: update fee rate units, use sat/vB for fee_rate error messages

and BTC/kvB for feeRate error messages.

test/functional/wallet_send.py

@@ -306,11 +306,10 @@ class WalletSendTest(BitcoinTestFramework):
                 self.test_send(from_wallet=w0, to_wallet=w1, amount=1, conf_target=v, estimate_mode=mode,
                     expect_error=(-3, "Expected type number for conf_target, got {}".format(k)))
 
-        # TODO: The error message should use sat/vB units instead of BTC/kB.
         # Test setting explicit fee rate just below the minimum.
         self.log.info("Explicit fee rate raises RPC error 'fee rate too low' if fee_rate of 0.99999999 is passed")
         self.test_send(from_wallet=w0, to_wallet=w1, amount=1, fee_rate=0.99999999,
-            expect_error=(-4, "Fee rate (0.00000999 BTC/kB) is lower than the minimum fee rate setting (0.00001000 BTC/kB)"))
+            expect_error=(-4, "Fee rate (0.999 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)"))
 
         # TODO: Return hex if fee rate is below -maxmempool
         # res = self.test_send(from_wallet=w0, to_wallet=w1, amount=1, conf_target=0.1, estimate_mode="sat/b", add_to_wallet=False)