We see many cases where documents have charts with a difference here, so
downgrade the exception to a warning for now
Also handle cloning charts in workbooks where the parts have unexpected names
by making sure that the chosen part-name is not yet taken.
git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1894676 13f79535-47bb-0310-9956-ffa450edef68
Dominik Stadler