src/usermod.c: -U: Report E_PASSWORDLESS on error due to passwordless account

src/usermod.c: -U: Report E_PASSWORDLESS on error due to passwordless account

Reproducer:

	$ useradd foo
	$ grep foo /etc/passwd /etc/shadow
	/etc/passwd:foo:x:1001:1001::/home/foo:/usr/bin/bash
	/etc/shadow:foo:!:20458:0:99999:7:::
	$ usermod -U testuser
	usermod: unlocking the user's password would result in a passwordless account.
	You should set a password with usermod -p to unlock this user's password.
	$ echo $?
	0
	$ grep foo /etc/passwd /etc/shadow
	/etc/passwd:foo:x:1001:1001::/home/foo:/usr/bin/bash
	/etc/shadow:foo:!:20458:0:99999:7:::

The program failed (didn't change anything, and reported the problem to
stderr) but reported success (0).  After this patch, the error is
reported as E_PASSWORDLESS (20).

Closes: <https://github.com/shadow-maint/shadow/issues/1479>
Reported-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Acked-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>