hi ,I was facing same error today, did same changes as given in previous post.
didn`t got anything.
after I did changes in manifest.xml in eclipse as suggested in Adobe Extension Builder and Creative Cloud 2014
this link and saved changes.
and run the project error occurs right .
this is due to garbled manifest.
if we see the manifest now it is changed all contents are missing.
entire extension tag is missing.
so to solve this don`t change manifest from within eclipse.
go to the ur workspace>project>\.staged-extension\CSXS\manifest.xml
make the same changes here ,save file.
and now you are good to develope your extension.