Don't exit in cleanup_and_fail

[ludeeus]

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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies: 3.10 +

• Issues fixed (if any):

• Brief description of code changes (suitable for use as a commit message):

#1146 which aimed to add the error message inside the JSON output did so by invoking iperf_errexit.
While this works it also has the side effect of terminating the parent process when the test run is invoked from python like with https://github.com/thiezn/iperf3-python

This PR provides the desired output that was requested in #1143 without exiting during cleanup.

[matt9j]

I independently ran into this issue, and was preparing a very similar PR when I encountered #1205 and the link to this commit. Fixing this is essential for any external usage of libiperf. Let me know if there is anything I can do to help test or validate changes!