The png library uses setjmp/longjmp to throw exceptions when reading. If this
is not set up, the png library calls abort(). This change handles the errors
and throws a C++ exception instead.
This issue was found by testing images from pngsuite at
http://www.schaik.com/pngsuite/. These images are included and a unit test was
added to test both images that should be successful and images that should
throw an exception.
