clean up reva ocdav webdav handler error handling

[butonic]

Currently, the reva ocdav webdav handler followcs a mixed approach to error handling:

		case MethodLock:
			status, err = s.handleLock(w, r, ns)
			...
		case MethodMove:
			s.handlePathMove(w, r, ns)
			...
		default:
			w.WriteHeader(http.StatusNotFound)
		}

		if status != 0 { // 0 means the handler already sent the response
			w.WriteHeader(status)
			if status != http.StatusNoContent {
				var b []byte
				if b, err = errors.Marshal(status, err.Error(), ""); err == nil {
					_, err = w.Write(b)
				}
			}
		}
		if err != nil {
			appctx.GetLogger(r.Context()).Error().Err(err).Msg(err.Error())
		}

All handlers should return a status and err, so the generic error handling can take place. In case the handler needs to handle a special case it can devactivate the generic handling by returning status = 0.

This will prevent and fix bugs where we don't render the xml body in an error case, eg:

• send (MOVE,COPY) requests to another user's webDav endpoints as normal user gives 400 instead of 404 #3882
• Unauthorized PROPFIND request to public link share responds with empty body #1945
• Ability to return error messages in Webdav response bodies #1293

It will make returning a special message easier, eg. for #773