Cleanly handle volumes missing slices

[csparker247]

Since the volume is lazy loaded at runtime, missing slices can cause a number of unexpected crashes across our various programs. Need to review the current behavior on image load and its effect on the apps. Might also want to add a verification util or something.

[csparker247]

#18 will keep VC from crashing, but this doesn't really handle any of the other edge cases. Historically we've considered a missing slice a hard error since it could be a missing file, data corruption, etc.

Since the current paradigm seems to be leaning towards partial volume downloads, the behavior here needs to be rethought. In particular, we want to know when slices are missing and decide how to handle them based on context. A segmentation algorithm should probably stop when it's missing a slice, for instance, but a GUI might not care.