Make collection validation more powerful

Handle and use exceptions.
An example is provided with the redmine backend (not very useful
though). If you cd into the project title instead of the id, it is
accepted and the path is corrected.

weboob/capabilities/collection.py

@@ -82,22 +82,21 @@ class ICapCollection(IBaseCap):
         it should return None.
         """
         collection = Collection(split_path, None, self.name)
-        if self._is_collection_valid(objs, collection.split_path):
-            return collection
+        return self.validate_collection(objs, collection) or collection
 
-    def _is_collection_valid(self, objs, split_path):
+    def validate_collection(self, objs, collection):
         """
         Tests if a collection is valid.
         For compatibility reasons, and to provide a default way, it checks if
         the collection has at least one object in it. However, it is not very
         efficient or exact, and you are encouraged to override this method.
+        You can replace the collection object entirely by returning a new one.
         """
         # Root
-        if len(split_path) == 0:
-            return True
+        if len(collection.split_path) == 0:
+            return
         try:
-            i = self.iter_resources(objs, split_path)
+            i = self.iter_resources(objs, collection.split_path)
             i.next()
-            return True
-        except (StopIteration, CollectionNotFound):
-            return False
+        except StopIteration:
+            raise CollectionNotFound(collection.split_path)