RpcError exception is not raised when <rpc-error> element is not in the netconf namespace.

[stacywsmith]

While the primary issue reported in #715 is a Junos bug, it did expose an issue with PyEZ. In #715, the response contains an <rpc-error> element, but no RpcError exception was raised.

Normally, ncclient raises an RPCError exception when an <rpc-error> tag is present in the response. PyEZ then catches this exception and raises its own RpcError exception.

However, ncclient only raises an RPCError exception when the <rpc-error> element is in the urn:ietf:params:xml:ns:netconf:base:1.0 namespace.

In this case, the full response from the device is:

<rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" xmlns:junos="http://xml.juniper.net/junos/16.1R4/junos" xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="urn:uuid:2032f515-3f87-4bce-9fe7-747bdba60ca5">
    <ping-results xmlns="http://xml.juniper.net/junos/16.1R4/junos-probe-tests">
        <ping-failure>usage error</ping-failure>
        <rpc-error>
           <error-type>protocol</error-type>
           <error-tag>operation-failed</error-tag>
          <error-severity>error</error-severity>
          <source-daemon>ping</source-daemon>
          <error-message>
usage: ping [-ACNQRSXadfnqrvw] [-c count] [-i wait] [-g loose-gateway]
       [-l preload] [-p pattern] [-s packetsize] [-t tos]
       [-F source] [-G strict-gateway] [-I interface]
       [-Ji|r|4|6|I interface|U routing-instance|L logical-router]
       [-T ttl] host
          </error-message>
        </rpc-error>
    </ping-results>
</rpc-reply>

The <rpc-error> element does not contain an xmlns attribute. Therefore, it inherits its namespace from an ancestor element. In this case, that's the http://xml.juniper.net/junos/16.1R4/junos-probe-tests namespace from the <ping-results> element. This causes ncclient not to recognize this as an error.

PyEZ should be enhanced to recognize this non-compliant Junos-specific situation and continue to raise an RpcError exception in these cases.