RpcServer: return RpcError.InsufficientNetworkFee error where appropriate

[AnnaShaleva]

Describe the bug
After #815 merge, RpcError.InsufficientFunds (with code -511) is returned on the incoming transaction verification when:

1. Sender's balance is not enough to pay for all his transactions in the pool
2. Sender's balance is not enough to pay for the particular incoming transaction's fees

Whereas RPC errors proposal postulates that -511 Insufficient funds error should be returned only in case 1, i.e. when:

| -511 || Insufficient funds || Sender doesn't have enough GAS to pay for all currently pooled transactions.

And all other cases of insufficient sender's balance should be covered by generic -500 error:

| -500 || Unclassified verification error|| Anything that can't be expressed by other codes.

To Reproduce
Steps to reproduce the behavior:

1. Check the transaction verification code from core: https://github.com/neo-project/neo/blob/b05501af882a0d1f2a1a7841c6ddc4d0504e5fc1/src/Neo/Network/P2P/Payloads/Transaction.cs#L368-L395
2. Check the error conversion code on the RPC server side:

   neo-modules/src/RpcServer/RpcServer.Node.cs

   Lines 97 to 100 in 253a77e

   	case VerifyResult.InsufficientFunds:
   	{
   	throw new RpcException(RpcError.InsufficientFunds.WithData(reason.ToString()));
   	}

Expected behavior

1. We need to modify TransactionVerificationContext.CheckTransaction so that it returns some new special error, this error should be detached from VerifyResult.InsufficientFunds and ported to other components (ref. https://github.com/neo-project/neo/blob/b05501af882a0d1f2a1a7841c6ddc4d0504e5fc1/src/Neo/Network/P2P/Payloads/Transaction.cs#L368.). This special error should be converted to RpcError with -511 code on the RPC server side whereas the rest of VerifyResult.InsufficientFunds cases should be converted to RpcError with -500 code.
2. We may need to adjust error text of RpcError.ExpiredTransaction to make it more precise to reflect the proposal intention. Currently it's:

   neo-modules/src/RpcServer/RpcError.cs

   Line 63 in 253a77e

   public static readonly RpcError InsufficientFunds = new(-511, "Insufficient funds for fee");

Platform:

• Version: RpcServer, current master (253a77e).

Additional context
The same problem was recently fixed in NeoGo, see nspcc-dev/neo-go#3360. This issue is not critical, but eventually it should be fixed.

[roman-khimov]

That's not the way it was intended to be, sorry. See nspcc-dev/neo-go#3361. The problem with C# code/modules is that it never returns -504.

I'll improve neo-project/proposals#156 wording about -511 though.

[AnnaShaleva]

Good, then no changes from neo-modules or core side are required, thus I'm closing this issue as not planned.

[AnnaShaleva]

The problem with C# code/modules is that it never returns -504.

Although we may use this issue to enable -504 in neo-modules (and core).