WebDAV: Convert JCR Exceptions to proper WebDAV-Exception to provide correct HTTP status

JCR exceptions like PathNotFoundException, AccessDeniedException and so must be migrated to Webdav-exception at store impl to provider proper HTTP status codes in error case.

The following cases need handling:

1) AccessDeniedException - try to connect to a repository with a valid user from the perspective of the web-server (webapp authentication) but which is not a valid repository user.
2) when the client uses a path shorter than host/repository_name/workspace_name, some meaningful exceptions should be thrown (instead of the default JCR exceptions)