…stdout from training utils (Issue tesseract-ocr#3998)

Here's the extract/backport from my master branch:

- use tprintf() for all error/warning/info/debug-diagnostics output --> fprintf(stderr, msg) --> tprintf(msg). This also anticipates migrating tesseract to using the fmt library (C++20) as done by @stweil in a dev branch.
- all error messages are prefixed with 'ERROR: '. This enables users, f.e., to grep/filter the stderr/log using a regex like '/^EERROR: /' to filter out the error messages. Application code which overrides the tprintf() API is another example: there error messages can be easily picked up and sent to different monitor channels / GUI windows / etc. in environments where there's no stdout/stderr (MSWindows GUI) or where end users are not accustomed to watching stdout/stderr.
- ditto for 'WARNING: ' level messages.

CAVEAT: this is a manual transfer from my master branch + code review whilee I was at it. I hope I got them all... :-)

… course, one only sees it when scrolling through the pullreq after the fact.

…nalyse process

…aught by CI

…s expected. Cause: omission in backporting this from tprintf+fmt code. Caught by CI.