[MERGE] raise NoSuchId when get_file_sha1 invoked with bad file id

[John Arbash Meinel]

John Arbash Meinel has voted +1.
Status is now: Approved
Comment:
Always nice to get rid of an 'assert What should I do?'

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C46A11B59.5050109%40utoronto.ca%3E