NETCONF provider should raise more appropriate exceptions

[111pontes]

Somewhat related to #773

Current Behavior

XML output for rpc-error is used as exception message. For instance:

YServiceProviderError: <?xml version="1.0"?>
<rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="1">
  <rpc-error>
    <error-type>protocol</error-type>
    <error-tag>unknown-namespace</error-tag>
    <error-severity>error</error-severity>
    <error-info>
      <bad-element>netconf-state</bad-element>
      <bad-namespace>urn:ietf:params:xml:ns:yang:ietf-netconf-monitoring2</bad-namespace>
    </error-info>
  </rpc-error>
</rpc-reply>

Expected Behavior

NETCONF service provide should define more specific exceptions that reflect the different types of transport/rpc/protocol/application errors as defined in RFC 6241

Users should be able to catch more specific exceptions if desired and the exception message in any case should be human-friendly instead of direct XML output from an rpc-reply.

Steps to Reproduce

Initiate a NETCONF RPC that results in a rpc-error message.

System Information

$ python --version
Python 3.5.2
$ pip list | grep ydk
ydk                     0.7.1
ydk-models-cisco-ios-xr 6.3.2
ydk-models-ietf         0.1.5
$