[MERGE] Fix error handling in insert_data_stream (+ remove duplicate HeadsCache)

[Ian Clatworthy]

Ian Clatworthy has voted approve.
Status is now: Approved
Comment:
I'll merge this.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C1200414823.8209.4.camel%40nemo%3E