(#7244) Make autosign command treat all non-zero returns the same

Previously the autosign command exit status was checked for three
states: 0, 1, or other.  The latter two would both fail to autosign,
but the other would also raise an exception, so different results
at the client.  We noticed this because the acceptance test
failed on Solaris (which returns 255 for /bin/false, rather than 1).

There doesn't seem to be a compelling reason to treat the non-zero
exit codes differently, so this patch simply removes that distiniction.