png: Return error for truncated PNG files

When a PNG image couldn't be loaded at the end of a progressive
load operation, throw an error.

The error string is already translated as part of the BMP and
PNM loaders.

https://bugzilla.gnome.org/show_bug.cgi?id=524216
tests: Add test image from bug 524216

So we can check that an error is thrown.

https://bugzilla.gnome.org/show_bug.cgi?id=524216