[UI] Display flow execution logs and not only tasks logs in the execution logs tab

[loicmathieu]

Feature description

Related to #2517

When there is an error in a flowable task or an executable task (and maybe on other places inside the executor), the task and the execution is failed inside the executor with an exception. This causes the executor to emit flow logs with the exception message not task logs. As those logs are not attached to any tasks, they are not displayed inside the Logs tab of an execution and we must use the global Logs page to access them.

Flow logs should be visible inside the execution logs table and not only task-related logs.

EDIT: we want these logs to be stored in the log table and be displayed in the logs tab of the Executions page