fix: throw an Exception when GD can't decode the image data

[jimwins]

This throws a more descriptive exception when GD fails to decode the image, which is likely to be because GD doesn't have support for that image type. Before, it would just fail on the subsequent call to imagesx().

[SRWieZ]

Hi! Thanks for the contribution.

Before merging, can you provide an image that is failing so we can write a test for that?

[jimwins]

You don't need a specific image, you can just feed it any non-image data, like extract_size_and_pixels_with_gd("this will fail").

[SRWieZ]

Fair enough. I added tests. Thank you for your contribution!