lnwallet/wallet.go: clarify anchor chan error str

Improve 'ErrReservedValueInvalidated' error string to explain that the
error is triggered by a transaction that would deplete funds reserved for
potential future anchor channel closings (via CPFP)

Add hint that further details can be found in the debug log

Update strings in 'lntest/itest/log_error_whitelist.txt' correspondingly

lnwallet/wallet.go

@@ -59,7 +59,8 @@ var (
 	// transaction that would take the walletbalance below what we require
 	// to keep around to fee bump our open anchor channels.
 	ErrReservedValueInvalidated = errors.New("reserved wallet balance " +
-		"invalidated")
+		"invalidated: transaction would leave insufficient funds for " +
+		"fee bumping anchor channel closings (see debug log for details)")
 )
 
 // PsbtFundingRequired is a type that implements the error interface and