Update findComponentRoot and processUpdates warnings for SVG

I ran into a really difficult-to-debug issue with a React app with SVG
elements. Christopher sat down with me for a while and we finally figured out
that the browser was moving non-SVG elements out of a parent `<svg>` node which
triggered the `processUpdates()` invariant. This updates the error message
thrown from there to include this scenario in the possible issues list.