How to get the error message in robot framework?

I have created a script to get the error output

***Settings***
Resource           importsLib.robot
Suite Setup     Run Keywords
...             Initialize Test     AND
...             Register Keyword To Run On Failure    Failure Callback
Variables       OMG.yaml

and the keywords

***Keywords***
Failure Callback
     Capture Page Screenshot
     Log Source    loglevel=WARN

but the point is I need to get HTML error message when Back-end didn't send the value to Front-end element sometimes and I need to track the root the root causes of faults or problems.

Can you offer the best solution for this case?