tests: framework gracefully handle 'VppTransportShmemIOError'

tests: framework gracefully handle 'VppTransportShmemIOError'

Catches:
----
Traceback (most recent call last):
  File "/vpp/test/framework.py", line 593, in tearDown
    self.logger.info(self.vapi.ppcli("api trace save %s" % api_trace))
  File "/vpp/test/vpp_papi_provider.py", line 413, in ppcli
    return cli + "\n" + str(self.cli(cli))
  File "/vpp/test/vpp_papi_provider.py", line 402, in cli
    r = self.papi.cli_inband(cmd=cli)
  File "/vpp/src/vpp-api/python/vpp_papi/vpp_papi.py", line 100, in __call__
    return self._func(**kwargs)
  File "/vpp/src/vpp-api/python/vpp_papi/vpp_papi.py", line 414, in f
    return self._call_vpp(i, msg, multipart, **kwargs)
  File "/vpp/src/vpp-api/python/vpp_papi/vpp_papi.py", line 634, in _call_vpp
    msg = self.transport.read()
  File "/vpp/src/vpp-api/python/vpp_papi/vpp_transport_shmem.py", line 120, in read
    raise VppTransportShmemIOError(rv, 'vac_read failed')
VppTransportShmemIOError: [Errno -1] vac_read failed
----

Change-Id: I767e48c4d03081eb5df6a8aa67da7e192d25e4cc
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>