[MERGE] document when to use custom exception classes (was Move RemoteRepository caching logic to CachedParentsProvider)

[Robert Collins]

Robert Collins has voted tweak.
Status is now: Conditionally approved
Comment:
-depending on whether ``internal_error`` is set or not.  If we think 
it's our
+depending on whether ``internal_error`` is set or not.  f we think it's 
our

typo

+#. Otherwise, use ``BzrError``.
+

How about
#. Otherwise, use ``BzrError`` or ``BzrInternalError``.

-Rob

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3C4941AF93.7000501%40sourcefrog.net%3E
Project: Bazaar