All WebDAV write-method errors (PUT/DELETE/MOVE/COPY/MKCOL/...) now go to
the console-only system log instead of the in-app errors table and browser
log viewer, because golang.org/x/net/webdav embeds absolute server paths in
its messages. Operators still see full detail in the container/service logs.
A MKCOL against an already-existing collection is a benign probe used by
sync clients such as PhotoSync to test for a directory before creating it;
it returns 405 and is logged at debug rather than as an error.
7bea2dccfe