LT-15001: Fix duplicate css classes

Duplicate classes were causing crashes when exporting
dictionaries to XHTML.  In 15008, the user had edited
LexEntry.fwlayout by hand; in 15001, duplicates exist
in our default LexEntry.fwlayout.
Also update error message not to use generic ToString

Change-Id: I6dd452d1f43378d614b6456d20637fc094e929d3