ticketvote: Cast vote error format.

[lukebp]

The cast vote reply is defined as:

type CastVoteReply struct {
	Ticket  string `json:"ticket"`  // Ticket ID
	Receipt string `json:"receipt"` // Server signature of client signature

	// The follwing fields will only be present if an error occurred
	// while attempting to cast the vote.
	ErrorCode    VoteErrorT `json:"errorcode,omitempty"`
	ErrorContext string     `json:"errorcontext,omitempty"`
}

This means that the ErrorCode is 0 when the vote is cast succesfully and a 0 error code corresponds to VoteErrorInvalid. This is incorrect. The ErrorCode should be nil on success. The reply should contain a pointer to the error.

The error handling code in politeiavoter needs to be updated once this is fixed. The decrediton error handling should also be checked to see if it needs to be updated as well. This doesn't impact non-go clients because the fields are not included in the JSON.

This bug was introduced by #1180.