parse.c: don't print error message on callback rv > 0

parse.c: don't print error message on callback rv > 0

A callback return value < 0 means there was an error, so print
out an error message.  But a rv > 0 is used by the mount_unknown_fs
functions to say "we found the one we want, stop here."

Document this, and only print an error message if rv < 0.  Otherwise,

	lxc-create -B lvm --fstype ext3 -t ubuntu -n u1

will print an (innocuous) error message about being unable to parse
the config value 'ext3'.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>