JpegImporter: redirect libjpeg errors to Corrade::Utility::Error

[xqms]

I'm crawling a large dataset with a few faulty JPEGs - which I would like to ignore silently. However, libjpeg errors are currently directly printed to stderr. This PR redirects them to Corrage::Utility::Error, which can then be redirected by the user to capture any errors that might occur.

[mosra]

Hi, thanks a lot! 👍

For some reason this was already done for the JpegImageConverter. Not sure why I didn't do the exact same thing here too.

Looks like this code path is not tested either. Will merge this later today / tomorrow together with a test case.

[xqms]

For some reason this was already done for the JpegImageConverter. Not sure why I didn't do the exact same thing here too.

Ah ok, didn't see that. That implementation even looks slightly better with an integrated one-line error message, so maybe just take that one ;-)

[mosra]

Finally merged in ee69e5c. While adding the test cases for proper code coverage I discovered a handful of random bugs in all image importers (the PNG importer, especially) and, 20 commits later, these should be fixed as well.