executor: rework feature setup

Return failure reason from setup functions rather than crash.
This will provide better error messages, but also allow setup
w/o creating subprocesses which will be needed when we combine
fuzzer and executor.

Also close all resources created during setup.
This is also useful for in-process setup, but also should improve
chances of reproducing a bug with C reproducer. Currently leaked
file descriptors may disturb repro execution (e.g. it may act
on a wrong fd).

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