manager: fix error handling after failure to set up child

manager: fix error handling after failure to set up child

exec_child() is supposed to set *exit_status when returning failure.
Unfortunately, we didn't do that in two cases. The result would be:
- a bogus error message "Failed at step SUCCESS spawning foo: …",
- a bogus success exit status.

Bugs introduced in 390902012c5177b6b01bc634b2e9c704073d9e7d and
ad21e542b20f0fb292d1958d3a759bf3403522c2.

The code is reworked to add some asserts and not set exit_status in the caller
so that it's clearer (also to the compiler) that it needs to be set.