it would be convenient to have a such a file.
Is it possible to make it as a separate log file?
Change logs level to ERROR in DEVTEST/logging.properties file
I did it but it not generated separately log for errors.
or I need to look it in some other log file?
Yes to create a separate ERROR file... that would have to be an enhancement request.
I have marked abrsh01's response as correct, as this feature is not currently available and should be entered as an idea.
If you do decide to create an idea on this, please post the link here, and i will mark that as correct so that other users looking for this feature can vote on/participate in the idea. Otherwise, if the idea Rick pointed out below works to fulfill your request, please let us know and we can mark that as correct.
What's the use case for this? Rather than having a completely separate storage for error messages (which would lose context, and be a problem for triage of any issue), do you actually want to be able to see errors without other information interfering with your view? If this is the case, you might actually want a view filter, so vote up and look into this Log file analysis in Portal