sysupdated: Rearrange error logging a little bit

sysupdated: Rearrange error logging a little bit

First, this fixes a case where an error is logged twice at the LOG_ERR
level. Second, this goes through and raises the log level where
appropriate so that sysupdated is easier to debug. Finally, it replaces
EINVAL with EPROTO where appropriate, since EINVAL implies that the
caller passed some incorrect arguments (which is incorrect; sysupdate
passed some invalid JSON back to us)