Explain is_package AttributeError in find_package

When a PEP 302 import hook is used that doesn't implement .is_package()
an AttributeError is raised. This looks like a bug in Flask. This change
fixes that problem in the sense that it explains, that the
AttributeError is intentional.

CHANGES

@@ -9,6 +9,9 @@ Version 0.10.2
 (bugfix release, release date to be announced)
 
 - Fixed broken `test_appcontext_signals()` test case.
+- Raise an :exc:`AttributeError` in :func:`flask.helpers.find_package` with a
+  useful message explaining why it is raised when a PEP 302 import hook is used
+  without an `is_package()` method.
 
 Version 0.10.1
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