a pointer should not be compared to zero [coccinelle]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
translation: unify exec error messages

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
eject: remove a duplicate command name from error message

Let the err() print the command name.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>