Handle FailedToStartExecutableError in test_exit_before_synchronization

With --exit-before-sync, hived exits immediately before binding an HTTP
port. Under CI load with parallel test execution, this can race with
beekeepy's port discovery, causing FailedToStartExecutableError.

The fix catches this expected exception since the test only needs to
verify that the node is not running after the call, which is true
whether the run() succeeds or throws FailedToStartExecutableError.