sw: SwXFrame: fix gradient import problem:
sw: SwXFrame: fix gradient import problem:

BaseFrameProperties_Impl::FillBaseProperties: It may happen that
checkForUniqueItem returns an item with a name that already exists, if
the same gradient appears multiple times; don't crash with unhandled
ElementExistException then.
(can be reproduced with test3.docx from fdo#41466)
(regression from 1a3c90a292c7fc9060604151de9dc51eecf5b6a7)

Change-Id: Iace7af4c5679c1c7e978525fa31abf884038567c

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core