Add checks for empty arrays post-segmentation

[joshuacwnewton]

Description

This PR provides more intuitive feedback when sct_deepseg_sc produces an empty array. This avoids a more unintuitive error later on in the postprocessing function.

Linked issues

Fixes #4197.

[mguaypaq]

These error messages look much better, thanks.

Since they're uncaught exceptions, they'll appear to the user with a possibly-confusing traceback. Could you add a try...except block in the CLI layer to instead display the associated error message using printv(..., 'error')? For example:

spinalcordtoolbox/spinalcordtoolbox/scripts/sct_label_vertebrae.py

Lines 426 to 428 in d6aeaef

	except ValueError as e:
	printv(f'Vertebral detection failed: {e}', 1, 'error')
	sys.exit(1)

[mguaypaq]

This looks fine, thanks! And yes, if we're going to have some general-purpose custom exceptions, types.py seems like a fine place for them. I think, as time goes on, we'll see whether or not we like this error-handling pattern.