PNG export: Exception on too few axes

[Lestropie]

Supersedes #2465.

This section of code is also relevant to #2179.

As described in this comment, the usage resulting in a segfault is likely not that intended. But further, attempting to save to out-[].png from a 1 x 128 x 128 image could (theoretically at least) yield either a single 128 x 128 image (effectively "ignoring" the square-bracket syntax), or, more faithfully, 128 different 1 x 128 images. Presence of unity axes is already used within the PNG handling to identify the slice orientation to use for PNG files.

For now, this comment instead catches the likely erroneous but also potentially ambiguous usage and errors out, rather than segfaulting. Later, hopefully for 3.1.0, I'll try to restructure the relevant code in order to address #2179, and hopefully if the code is better structured the right solution in this instance will fall out.

[Lestropie]

Should also note that unlike #2465 this is better guaranteed to not change behaviour on master under alternative use cases.

[jdtournier]

Seems to catch the problematic usage on my system. 👍