pixma_bjnp: Report that scanner model is not supported as suggested in issue #69

Going through all issues that Olaf changed today, I found a comment in issue #69 (closed) that suggested to report unsupported scanners.

There was a very unclear debug statement in pixma_io_sane that reports this in the pixma log (I am still looking at that, maybe move it to the bjnp log if possible) but that is a detailed debug log. So I added a critical error report in pixma_bjnp.c Please suggest if we should include this for 1.0.29.

Updated the patch for better debugging support.