growfs: make exit codes more consistent

We have overused err(1), so it's hard to distinguish when an error is
very, very serious, and when it's just a user-error, or even harmless.

This patch changes the current behaviour to distinguish between the
following three:

1 for usage errors
2 for recoverable errors
3 or higher for unrecoverable errors

Reviewed by: jilles, pauamma_gundo.com, des
Differential Revision: https://reviews.freebsd.org/D27161