ionice: non-cryptic error message when ionice can't execvp

ionice: non-cryptic error message when ionice can't execvp

Previously ionice would reply like this when it couldn't find the
command to execute:

    $ ionice -c 3 does-not-exist
    ionice: execvp failed: No such file or directory

Getting that message from cron because you haven't set the right $PATH
makes it hard to track down the problem. Now it says:

    $ ionice -c 3 does-not-exist
    ionice: executing does-not-exist failed: No such file or directory

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>