fix: wait_for_termination returns actual exit status

[sweb]

Description

since set -e is the last part of wait_for_termination it always has exit code 0. With this, it returns the exit code of the process it is waiting for instead. [ "${term_child_pid}" ] blows up when the child pid is not set yet, so gets changed to [ -n "${term_child_pid:-}" ].

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[lfrancke]

Ah...we just had the same/similar in stackabletech/stackable-utils#130

It's an annoying trap.

[sweb]

Ah...we just had the same/similar in stackabletech/stackable-utils#130

It's an annoying trap.

in this context, it is not a huge deal - from what I checked it does not interfere with OOM based restarts or something like that - but getting the actual exit codes may be nice and should be an improvement.