Consistent case for error prefix

[BenBE]

In error messages the case for writing error should be consistent. There seem to be places with ERROR:, Error: and error: in the code. Maybe we should decide for one of those variants and use it consistently.

[phlash]

We might be at the mercy of various libraries - I note with distaste that many parts of OpenCV print directly to stderr rather than use the provided error logging facility (which applications can intercept/format etc.) - but yep, our stuff should be consistent 😄

[BenBE]

For htop the stderr is completely redirected into a memfd and only ever dumped to the real stderr on abnormal exit. We may think about doing something similar here too. But that is another issue if we decide to go that route.