tests: improve vpp_papi_provider exception output

tests: improve vpp_papi_provider exception output

saves time debugging tests by replacing 'API call' with
the actual function signature:

  vpp_papi_provider.UnexpectedApiReturnValueError: API call failed, expected 0 return value instead of -2 in vxlan_add_del_tunnel_reply(_0=247, context=5052, retval=-2, sw_if_index=4294967295)

  vpp_papi_provider.UnexpectedApiReturnValueError: vxlan_add_del_tunnel(is_add=0, src_address=172.16.1.1, dst_address=239.1.1.209, vni=209, sw_if_index=26, mcast_sw_if_index=1, encap_vrf_id=None, instance=None, decap_next_index=None) failed, expected 0 return value instead of -2 in vxlan_add_del_tunnel_reply(_0=247, context=5052, retval=-2, sw_if_index=4294967295)

Type: test

Change-Id: Ie3b6a5fdb4e1d427d60c51f7a7bf815af0bb3de6
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>