add handling of new dynamic address registration errors in PAN-OS 6.0.

Hi Kevin,

I noticed that in PAN-OS 6.0, if you tag an IP with a tag it already has, it produces an error which is raised as a PanXapiError.  This is fine, but the error text is blank so it's impossible to tell this PanXapiError from another.  In my code, I needed to ignore this error because I don't care if the tag already existed, so I needed a way to distinguish this error from others.  The XML response for the error looks like this:
```
<response status="error">
    <msg>
        <line>
            <uid-response>
                <version>2.0</version>
                <payload>
                    <register>
                        <entry ip="192.168.1.2" message="tag your-tag already exists, ignore"/>
                        <entry ip="192.168.1.3" message="tag your-tag already exists, ignore"/>
                    </register>
                </payload>
            </uid-response>
        </line>
    </msg>
</response>
```
I added some code to the function that gets the return status in xapi so that it can handle this new `<uid-response>` tag to get the error message correctly.  It only handles register and unregister errors, but these are the only errors I've seen from the User-ID API that aren't handled.

Feel free to update or enhance my attempt as you see fit.  Thanks!