Do not crash on swsssdk.exceptions.UnavailableDataError

[stcheng]

Should have a better way of handling such case.

Traceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/local/lib/python3.6/dist-packages/sonic_ax_impl/__main__.py", line 74, in <module>
    from .main import main
  File "/usr/local/lib/python3.6/dist-packages/sonic_ax_impl/main.py", line 14, in <module>
    from .mibs.ietf import rfc1213, rfc2863, rfc4292, rfc4363
  File "/usr/local/lib/python3.6/dist-packages/sonic_ax_impl/mibs/ietf/rfc1213.py", line 337, in <module>
    class InterfacesMIB(metaclass=MIBMeta, prefix='.1.3.6.1.2.1.2'):
  File "/usr/local/lib/python3.6/dist-packages/sonic_ax_impl/mibs/ietf/rfc1213.py", line 342, in InterfacesMIB
    if_updater = InterfacesUpdater()
  File "/usr/local/lib/python3.6/dist-packages/sonic_ax_impl/mibs/ietf/rfc1213.py", line 157, in __init__
    self.update_data()
  File "/usr/local/lib/python3.6/dist-packages/sonic_ax_impl/mibs/ietf/rfc1213.py", line 176, in update_data
    for sai_id in self.if_id_map}
  File "/usr/local/lib/python3.6/dist-packages/sonic_ax_impl/mibs/ietf/rfc1213.py", line 176, in <dictcomp>
    for sai_id in self.if_id_map}
  File "/usr/local/lib/python3.6/dist-packages/swsssdk/interface.py", line 38, in wrapped
    ret_data = f(inst, db_name, *args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/swsssdk/interface.py", line 303, in get_all
    raise UnavailableDataError(message, _hash)
swsssdk.exceptions.UnavailableDataError: Key 'b'COUNTERS:oid:0x1000000000002'' unavailable in database 'COUNTERS_DB'