Print detailed error messages for failed operations

[jonasob]

Seems to work reasonably well, but I've found a PNG file on which it breaks. Might be good to figure out at least why it breaks on this one :)

jonas@dev:~$ blockhash/build/blockhash EZSYG.png
Error opening image file EZSYG.png

(it doesn't actually have a problem opening it, but it seems this is the error thrown for any error parsing a file once opened)

[artfwo]

This file is corrupt (a CRC error). We can provide error details for such images, though, so I'm renaming the issue accordingly.