Regular Python exceptions are explicitly raised at various spots in the codebase.

Better to refactor these to (possibly new) subclasses of SphobjinvError.

• "CountError" instead of ValueError or ValueError
• Zero-length inventory error instead of ValueError
• contract and expand both True error, instead of ValueError

Make sure to remove any per-file flake8 error ignores for cmdline.py once the exception handling is (presumably) de-generalized.

Also likely add flake8-errmsg to the flake8 plugins.