Fix getImage(s)Blob() return type

[HypeMC]

PR #616 defined the return type of getImageBlob() as nullable, but that is no longer correct because PR #606 changed it to throw an exception instead of returning null.

This PR sets the return type of getImageBlob() back to non-nullable and updates getImagesBlob() to throw an exception when it fails to get the image contents. This makes both methods behave consistently.

[bukka]

The getImageBlob type change is correct but I'm wondering if we should rather change type back for getImagesBlob to nullable rather than throwing an exception. There might be some code in old version relying on null being returned so we could consider it as a bug.

[HypeMC]

I'm wondering if we should rather change type back for getImagesBlob to nullable rather than throwing an exception. There might be some code in old version relying on null being returned so we could consider it as a bug.

@bukka I'm fine either way, but the change to getImageBlob was introduced in 3.8.0, so the same argument could be made for anyone updating from 3.7.0. IMO, having consistent behavior that aligns with the docs is the better choice, but I'm happy to change it if you want.

[bukka]

Good point about the docs. It probably makes sense. I will wait few weeks and if there are no objections, I will merge it.

[bukka]

Pipeline failure is unrelated - looks like 6.9.2-0 is missing in launchpad