Use stderr for errors

[andreiburdusa]

Some error messages were being printed to standard output. This could have caused them to be printed in the output file if the user used the flag --output, thus being mixed with the kore pattern.

The present pull request replaces putStrLn with hPutStrLn stderr in those places.

---

Review checklist

The author performs the actions on the checklist. The reviewer evaluates the work and checks the boxes as they are completed.

• Summary. Write a summary of the changes. Explain what you did to fix the issue, and why you did it. Present the changes in a logical order. Instead of writing a summary in the pull request, you may push a clean Git history.
• Documentation. Write documentation for new functions. Update documentation for functions that changed, or complete documentation where it is missing.
• Tests. Write unit tests for every change. Write the unit tests that were missing before the changes. Include any examples from the reported issue as integration tests.
• Clean up. The changes are already clean. Clean up anything near the changes that you noticed while working. This does not mean only spatially near the changes, but logically near: any code that interacts with the changes!

[ttuegel]

The code looks good. Could you please add to the pull request summary why it is important to use stderr instead of stdout?