[MERGE][#133782] Better error message when running ``bzr cat`` on a non-existant branch.

[Aaron Bentley]

Aaron Bentley has voted comment.
Status is now: Semi-approved
Comment:
I think it doesn't make sense to catch NotBranchError and pass-- if 
we're not referring to a file in a branch, then what are we referring 
to?

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C1189253358.6645.35.camel%40nemo%3E