[MERGE] Define error for Repository.revision_tree()

[Aaron Bentley]

Aaron Bentley has voted +1.
Status is now: Approved
Comment:
Seems good to me.  I think that these errors are essentially the same 
thing, because "NoSuchRevision" really means "No such revision *in this 
context*".  We can never know whether a given revision exists in another 
context-- that's the blessing and curse of being decentralized.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C45D6718A.5060801%40arbash-meinel.com%3E