multi: update blockchain and mempool error types.

[dnldd]

This updates the blockchain and mempool error types to leverage go 1.13 errors.Is/As functionality as well as confirm to
the error infrastructure best practices.

[davecgh]

Thanks for the PR. It looks pretty good overall. There are a couple of incorrect cases I identified inline as well as many opportunities to simplify the code with by making full use of the new infrastructure also identified inline.

[davecgh]

Round 1 of latest updates.

[davecgh]

This is still not really correct. Now that both blockchain and mempool use the new error infrastructure, I'd suggest simplifying all this to just be.

func errToWireRejectCode(err error) (wire.RejectCode, string) {
	// The default reason to reject a transaction/block is due to it being
	// invalid somehow.
	code := wire.RejectInvalid
	var reason string

	// Convert recognized errors to a reject code.
	switch {
	// Rejected due to duplicate.
	case errors.Is(err, blockchain.ErrDuplicateBlock):
		code = wire.RejectDuplicate
		reason = err.Error()

	// Rejected due to obsolete version.
	case errors.Is(err, blockchain.ErrBlockVersionTooOld):
		code = wire.RejectObsolete
		reason = err.Error()

	// Rejected due to checkpoint.
	case errors.Is .... the rest

	...

	// Error codes which map to a duplicate transaction .....
	case errors.Is(err, mempool.ErrMempoolDoubleSpend),
		errors.Is(err, mempool.ErrAlreadyVoted),
		.... the rest:

		code = wire.RejectCheckpoint
		reason = err.Error()

	default:
		reason = fmt.Sprintf("rejected: %v", err)
	}
	return code, reason
}

In other words, there is no longer a need to manually unwrap and check things. Just check for the recognized things directly and let errors.Is handle the unwrapping.