Update NXOS response processing for more informative error messaging

[dkaplan1]

This PR addresses improvements that can be made to how nxapi_plumbing processes JSON-RPC responses. The issue stems from different behavior in JSON-RPC vs XML responses, specifically return codes. JSON-RPC returns a 500 for an erroneous command, whereas XML returns a 200.

JSON

(Pdb) print(response)
<Response [500]>
(Pdb) print(response.json())
{'jsonrpc': '2.0', 'error': {'code': -32602, 'message': 'Invalid params', 'data': {'msg': 'SNMP community entry not found.\n'}}, 'id': 1}

XML

(Pdb) print(response)
<Response [200]>
(Pdb) print(response.text)
<?xml version="1.0"?>
<ins_api>
  <type>cli_conf</type>
  <version>1.0</version>
  <sid>eoc</sid>
  <outputs>
    <output>
      <clierror>SNMP community entry not found.
</clierror>
      <code>400</code>
      <msg>CLI execution error</msg>
    </output>
  </outputs>
</ins_api>

This means that erroneous commands using JSON-RPC will always raise NXAPIPostError with a generic error message including all commands with no indication of the underlying issue.

napalm/napalm/nxapi_plumbing/api_client.py

Lines 81 to 87 in d72e194

	if response.status_code not in [200]:
	msg = """Invalid status code returned on NX-API POST
	commands: {}
	status_code: {}""".format(
	commands, response.status_code
	)
	raise NXAPIPostError(msg)

napalm.nxapi_plumbing.errors.NXAPIPostError: Invalid status code returned on NX-API POST
commands: [...]
status_code: 500

If we move non-200 response raising to XMLClient's _process_api_response, erroneous JSON-RPC requests will be returned and passed to _error_check, which will give better insight into what the problematic command was and information on the issue.

napalm.nxapi_plumbing.errors.NXAPICommandError: The command "..." gave the error "..."

In order to test this I had to make a few changes to plumb exposure of the status code.

Also, while going through the NXOS tests I noticed that we weren't raising when mocked_data included exception directives, so things like

napalm/test/nxos/mocked_data/test_get_interfaces_ip/no_ipv6_support/show_ipv6_interface.json

Lines 1 to 3 in d72e194

	{
	"exception": "nxapi_plumbing.api_client.NXAPIPostError"
	}

were not working as expected.