This commit makes possible to pass an optional `X-Garg-Error-Scheme`
header in a client request to enable the new error format.

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).