Don't throw exceptions on unprocessed hrefs in get_multi

Rather, just log them. This allows my calendar sync to continue to work
even if some of the hrefs can't be retrieved. In my case, one of the
hrefs on my calendar was always returning a 404, so I want to skip it.

vdirsyncer/storage/dav.py

@@ -547,7 +547,7 @@ async def get_multi(self, hrefs):
                 else:
                     rv.append((href, Item(raw), etag))
             for href in hrefs_left:
-                raise exceptions.NotFoundError(href)
+                dav_logger.warning(f"Skipping {href}, not found")
 
             for href, item, etag in rv:
                 yield href, item, etag
@@ -645,6 +645,7 @@ def _parse_prop_responses(self, root, handled_hrefs=None):
             props = response.findall("{DAV:}propstat/{DAV:}prop")
             if props is None or not len(props):
                 dav_logger.debug(f"Skipping {href!r}, properties are missing.")
+                dav_logger.debug(f"Response for {href!r}: {etree.tostring(response)}")
                 continue
             else:
                 props = _merge_xml(props)