Use SystemExit not sys.exit & only on error paths.

[kousu]

Suggestion for #2982

[joshuacwnewton]

It looks like isct_convert_binary_to_trilinear -h fails because we didn't exit after the other usage() calls.

Traceback (most recent call last):
  File "/home/joshua/Repos/spinalcordtoolbox/scripts/isct_convert_binary_to_trilinear.py", line 196, in <module>
    main()
  File "/home/joshua/Repos/spinalcordtoolbox/scripts/isct_convert_binary_to_trilinear.py", line 92, in main
    check_file_exist(fname_data, verbose)
  File "/home/joshua/Repos/spinalcordtoolbox/spinalcordtoolbox/utils/fs.py", line 109, in check_file_exist
    if fname[0] == '-':
IndexError: string index out of range

[kousu]

@joshuacwnewton i think i got it this time.
but btw i made this a PR into your PR so either you have to reenable yours and we need to do the two merges or I need to close this and resubmit it pointed at master. Can we do the former?

[joshuacwnewton]

but btw i made this a PR into your PR so either you have to reenable yours and we need to do the two merges or I need to close this and resubmit it pointed at master. Can we do the former?

Terribly sorry for not noticing that! I'll rebase+merge and we can continue discussion there. :)