[MM-37557] Move error out of client4 response (#18101)

* Return an error seperately from Response

* Remove BuildErrorResponse

* Drop Response.Error from model/client4.go

* Migrate require.Nil checks

* Migrate require.NotNil checks

* More manual fixes

* Move error check out of CheckOKStatus and CheckCreatedStatus

* Move error check out of CheckForbiddenStatus

* Move error check out of CheckUnauthorizedStatus

* Move error check out of CheckNotFoundStatus

* Move error check out of CheckBadRequestStatus

* Move error check out of CheckNotImplementedStatus and CheckRequestEntityTooLargeStatus

* Move error check out of CheckInternalErrorStatus

* Move error check out of CheckServiceUnavailableStatus

* Remove error check from checkHTTPStatus

* Remove remaining references to Response.Error

* Check previously unchecked errors

* Manually fix compile and linter errors

* Return error in CreateWebSocket methods

* Return error instead of *AppError in DoApi methods

* Manually fix bad replacments

* Conistently return Response and error

* Use err instead of seperate bool return value to indicate success

* Reduce ussage of model.AppError in web/oauth_test.go

* Remove client4.Must

* Check error in buf.ReadFrom

* Fix failing tests

model/websocket_client.go

@@ -59,13 +59,13 @@ type WebSocketClient struct {
 
 // NewWebSocketClient constructs a new WebSocket client with convenience
 // methods for talking to the server.
-func NewWebSocketClient(url, authToken string) (*WebSocketClient, *AppError) {
+func NewWebSocketClient(url, authToken string) (*WebSocketClient, error) {
 	return NewWebSocketClientWithDialer(websocket.DefaultDialer, url, authToken)
 }
 
 // NewWebSocketClientWithDialer constructs a new WebSocket client with convenience
 // methods for talking to the server using a custom dialer.
-func NewWebSocketClientWithDialer(dialer *websocket.Dialer, url, authToken string) (*WebSocketClient, *AppError) {
+func NewWebSocketClientWithDialer(dialer *websocket.Dialer, url, authToken string) (*WebSocketClient, error) {
 	conn, _, err := dialer.Dial(url+ApiUrlSuffix+"/websocket", nil)
 	if err != nil {
 		return nil, NewAppError("NewWebSocketClient", "model.websocket_client.connect_fail.app_error", nil, err.Error(), http.StatusInternalServerError)
@@ -97,13 +97,13 @@ func NewWebSocketClientWithDialer(dialer *websocket.Dialer, url, authToken strin
 
 // NewWebSocketClient4 constructs a new WebSocket client with convenience
 // methods for talking to the server. Uses the v4 endpoint.
-func NewWebSocketClient4(url, authToken string) (*WebSocketClient, *AppError) {
+func NewWebSocketClient4(url, authToken string) (*WebSocketClient, error) {
 	return NewWebSocketClient4WithDialer(websocket.DefaultDialer, url, authToken)
 }
 
 // NewWebSocketClient4WithDialer constructs a new WebSocket client with convenience
 // methods for talking to the server using a custom dialer. Uses the v4 endpoint.
-func NewWebSocketClient4WithDialer(dialer *websocket.Dialer, url, authToken string) (*WebSocketClient, *AppError) {
+func NewWebSocketClient4WithDialer(dialer *websocket.Dialer, url, authToken string) (*WebSocketClient, error) {
 	return NewWebSocketClientWithDialer(dialer, url, authToken)
 }