DAOS-15031 test: launch.py: exit early if extra yaml is not a file

[daltonbohning]

Test-tag: test_always_passes
Skip-unit-tests: true
Skip-fault-injection-test: true

Exit as early as possible if extra yaml is not a file.

Required-githooks: true

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[github-actions]

Bug-tracker data:
Ticket title is 'launch.py: report clean error if extra yaml does not exist'
Status is 'In Progress'
Labels: 'test_infra'
https://daosio.atlassian.net/browse/DAOS-15031

[daltonbohning]

@phender The benefit of this approach is that it exits as early as possible and is simple. But it will not create a job.log in the event of failure. E.g.

$ ./launch.py -e fake
...
launcher.py: error: argument -e/--extra_yaml: File not found: fake

But this is inline with other invalid arguments. E.g.

$ ./launch.py -tm not-a-float
launcher.py: error: argument -tm/--timeout_multiplier: invalid float value: 'not-a-float'

[daosbuild1]

Test stage Functional Hardware Large completed with status FAILURE. https://build.hpdd.intel.com//job/daos-stack/job/daos/view/change-requests/job/PR-13624/7/execution/node/973/log

[daltonbohning]

soak_smoke failure is
https://daosio.atlassian.net/browse/DAOS-14548

[daosbuild1]

Test stage Functional Hardware Large completed with status FAILURE. https://build.hpdd.intel.com//job/daos-stack/job/daos/view/change-requests/job/PR-13624/10/execution/node/974/log