isosize: move file name printing after error determination

Earlier the filename printing was buffered, and exit at error made output
to appear in front of prompt.  Output below demonstrates the brokenness.

prompt> isosize /etc /
isosize: /etc: might not be an ISO filesystem
isosize: read error on /etc: Is a directory
/etc: prompt>

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
tests: add isosize --sectors and --divisor tests

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
isosize: make --divisor to require argument

Silly bug, only the short option -d allowed divisor argument.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>