[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

api4/command_help_test.go

@@ -15,7 +15,7 @@ func TestHelpCommand(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
 
-	Client := th.Client
+	client := th.Client
 	channel := th.BasicChannel
 
 	HelpLink := *th.App.Config().SupportSettings.HelpLink
@@ -24,12 +24,12 @@ func TestHelpCommand(t *testing.T) {
 	}()
 
 	th.App.UpdateConfig(func(cfg *model.Config) { *cfg.SupportSettings.HelpLink = "" })
-	rs1, _ := Client.ExecuteCommand(channel.Id, "/help ")
+	rs1, _, _ := client.ExecuteCommand(channel.Id, "/help ")
 	assert.Equal(t, rs1.GotoLocation, model.SupportSettingsDefaultHelpLink, "failed to default help link")
 
 	th.App.UpdateConfig(func(cfg *model.Config) {
 		*cfg.SupportSettings.HelpLink = "https://docs.mattermost.com/guides/user.html"
 	})
-	rs2, _ := Client.ExecuteCommand(channel.Id, "/help ")
+	rs2, _, _ := client.ExecuteCommand(channel.Id, "/help ")
 	assert.Equal(t, rs2.GotoLocation, "https://docs.mattermost.com/guides/user.html", "failed to help link")
 }