ZTS: fail test run if test runner crashes unexpectedly

zfs-tests.sh executes test-runner.py to do the actual test work. Any
exit code < 4 is interpreted as success, with the actual value
describing the outcome of the tests inside.

If a Python program crashes in some way (eg an uncaught exception), the
process exit code is 1.

Taken together, this means that test-runner.py can crash during setup,
but return a "success" error code to zfs-tests.sh, which will report and
exit 0. This in turn causes the CI runner to believe the test run
completed successfully.

This commit addresses this by making zfs-tests.sh interpret an exit code
of 255 as a failure in the runner itself. Then, in test-runner.py, the
"fail()" function defaults to a 255 return, and the main function gets
wrapped in a generic exception handler, which prints it and calls
fail().

All together, this should mean that any unexpected failure in the test
runner itself will be propagated out of zfs-tests.sh for CI or any other
calling program to deal with.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17858