This commit modifies the backend code to convert the input CSV into an
`NgramsList`, so that we can reuse the JSON API for the upload.

it also brings the CSV and JSON APIs closer in look.
Temporarily disable the `importCsvFile` function.

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.