A filter that can't become a live effect warned "filter on a group
ignored", even on a plain <rect>: usvg wraps every filtered element in
a group of its own, which the message named. The warning now names the
filter and its primitives and the element (by id, or "an object"), and
says it is drawn without it: "filter 'f' (feColorMatrix) on an object
isn't supported; drawn without it".
Fixes#1598