cephadm: catch Exception during is_mgr_available

cephadm: catch Exception during is_mgr_available

Traceback (most recent call last):
  File "test_tmp/tmp.ykMmPiFqIT/cephadm", line 4578, in <module>
    r = args.func()
  File "test_tmp/tmp.ykMmPiFqIT/cephadm", line 1121, in _default_image
    return func()
  File "test_tmp/tmp.ykMmPiFqIT/cephadm", line 2489, in command_bootstrap
    is_available('mgr', is_mgr_available)
  File "test_tmp/tmp.ykMmPiFqIT/cephadm", line 895, in is_available
    if func():
  File "test_tmp/tmp.ykMmPiFqIT/cephadm", line 2486, in is_mgr_available
    out = cli(['status', '-f', 'json-pretty'], timeout=timeout)
  File "test_tmp/tmp.ykMmPiFqIT/cephadm", line 2403, in cli
    return CephContainer(
  File "test_tmp/tmp.ykMmPiFqIT/cephadm", line 2140, in run
    out, _, _ = call_throws(
  File "test_tmp/tmp.ykMmPiFqIT/cephadm", line 838, in call_throws
    raise RuntimeError('Failed command: %s' % ' '.join(command))

Signed-off-by: Michael Fritch <mfritch@suse.com>