When the document contains DDE links to other documents, upon opening
you'll be asked whether you want to update the links.  When the
source documents are not open, however, the update fails, and all the
formula cells being updated become #N/A.  Not good.

With this change, when a DDE update fails, it skips updating the
formula cells that contains the DDE linkage (thus avoiding the #N/A
error) and instead, shows an error message telling the user that the
DDE link update has failed.