Checking this in middleware not only removes duplicated code from each of the handlers, but also makes things more efficient. Previously, dcrd/dcrwallet/database clients were initialized (and could possibly cause errors) **before** checking if the VSP is closed. Now the check happens before those other things.
Also bundled with some improvements to setaltsignaddr_test.go which I implemented along the way.
* Remove helper func getCommitmentAddress.
This function is only used in one place so removing it does not introduce any duplication. Removing the func also removes the need for errors.Is comparison, which will be very useful in upcoming changes.
* Rename apiError to ErrorCode.
* Don't use inline type for API errors.
* Export webapi errors.
* Export webapi request/responses.
* Add types module for API requests/responses/errors
`CanTicketVote` doesn't really fit into the RPC client code as it is more of a business logic function. Moving it into the webapi package is more appropriate.
