[MERGE] Move RemoteRepository caching logic to CachedParentsProvider

[John Arbash Meinel]

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Martin Pool wrote:
> On 11 Dec 2008, Aaron Bentley <aaron at aaronbentley.com> wrote:
> 
>>> I'd prefer just a plain AssertionError to further bloating errors.py.  I
>>> believe Martin has previously stated that we should try to avoid adding
>>> errors that we never intend to catch.
> 
> I have, and I think it still makes sense.  If we're just going to show
> it to a user as an error message, it doesn't really matter what class
> would be raised.  If code would plausibly want to catch it, to know what
> particular error was raised or what its parameters were, then it should
> have a new class.
> 

So the way I've interned your suggestions is.

1) If it is something that a caller can recover from, a custom exception
is reasonable.

2) If it is a data consistency issue, using a builtin like
ValueError/TypeError is reasonable.

3) If it is a programmer error (using an api incorrectly) AssertionError
is reasonable.

John
=:->

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