thanks for all suggestions,
googling for 'text grabber' and 'text scraping' I found more than 7 candidates for the job but none of them give me complete satisfaction.
Countchars and sysexporter are not bad but none of them work in all cases....
was surprised to learn that ctrl+c could do the job , I know it works in the calculator of windows for example to copy the result but so far I was not able to use ctrl+c to copy text from an error screen. I still believe it would be a nice addon for screenshot capture ..