meaningful error message for WebDAV upload of forbidden content

[Plone Forge]

When a user uses WebDAV, they login first and get Unauthorized errors if they fail.

If they've successfully logged in, and upload a bunch of files, they may
succeed on the .html files, and if there is a forbidden content type in the batch, such as .js, they also get an Unauthorized error which is confusing.

The patch changes Zope-2.9.8-final/lib/python/webdav/NullResource.py
to raise a more detailed Forbidden error in this kind of case.

[Jens Vagelpohl (dataflake-deactivatedaccount-deactivatedaccount)]

I have not changed the previous Unauthorized to a Forbidden as suggested by the patch due to backwards compatibility concerns with applications that might expect the 401 instead of the 403. The message changes are now checked in:

Zope 2.12 branch: http://svn.zope.org/?rev=113513&view=rev

Zope trunk: http://svn.zope.org/?rev=113514&view=rev