This commit starts adding the conversion functions from a
`BackendInternalError` to a `FrontendError`.

It was only for test for now.

Added more tests for more subtle cases.

Also, some fixes to NodeStory (it inserts ngrams when saving node
story, when they don't exist).

This commit tries to improve the `isSharedWith` SQL function to deal
with arbitrarily deep shared nodes in the hierarchy.