[BUG] improve error messages

[Aaron Bentley]

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John A Meinel wrote:

>>I don't think that anything in branch should throw a CommandError - it
>>hides too much information. Rather I think that the commands run()
>>should catch NotBranchError and convert that to a nice error message.
> 
> 
> Well, at first that is what I thought, too. But I was concerned that
> then *every* command would have to re-implement the same error trapping.
> What about having "find_branch_for_command()" which throws a better set
> of errors?

What I've done is implemented the rethrowing in bzrlib.commands.main.
That should avoid the code duplication.  The problem with
find_branch_for_command is, when do we use it, instead of find_branch?

Aaron
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