[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

app/plugin_api_test.go

@@ -1709,8 +1709,8 @@ func TestPluginHTTPUpgradeWebSocket(t *testing.T) {
 	require.NotEmpty(t, pluginID)
 
 	reqURL := fmt.Sprintf("ws://localhost:%d/plugins/%s", th.Server.ListenAddr.Port, pluginID)
-	wsc, appErr := model.NewWebSocketClient(reqURL, "")
-	require.Nil(t, appErr)
+	wsc, err := model.NewWebSocketClient(reqURL, "")
+	require.NoError(t, err)
 	require.NotNil(t, wsc)
 
 	wsc.Listen()