Improved unit session logging and notification

[rubenarslan]

New feature plan

• Improve logging of unit session results,
  • Log bugs (esp. in R) in more detail in log_result
  • Log runtime (i.e. just how many seconds a unit session took, including API calls to OpenCPU and elsewhere). This will make it easier to see resource-hungry studies and recurrent bugs, and to optimise code. Maybe simply wrap around UnitSession::execute?
    • Open question: what about unit_sessions that are executed multiple times (e.g., surveys, pauses). Simply add to runtime?
  • Dashboard for study admins and super admins to see runtime/compute use?
  • Configurable runtime/compute limits per user per month?
• ⁠⁠Notify study admins via email when errors occur during runtime
  • Emails should note name of run, affected run session and unit, error message
  • Shouldn't send emails in test sessions
  • Maybe a daily digest of errors with an ofelia cron job? Or notify for first five errors per day?