core: add missing error_message cases (#6911)

core: add missing error_message cases (#6911)

We neglected to set error_message for errors which occur _after_ the
`finish` label.  These fatal errors only happen in paths where `finish`
was reached successfully, i.e. error_message has not already been set
(and this analysis is simple enough that this need not cause too much
headaches.  Also our new assignments to error_message come immediately
after execve() calls, which would have lost the error_message if it had
been set).

Also print a status message when we fail to exec init, otherwise the only
sign the user will see is `# ` :).

This addresses the lack of error messages pointed out in issue #6827.