Warn users when text is too short

[dmlls]

The backend has been updated to the 0.0.2 version.

One of the changes is that now the backend will refuse to summarize very short texts (less than 20 words). The HTTP response in these cases is, e.g.:

422 Unprocesable Entity

{
    "detail": "source too short (<20 words)"
}

When this happens, the app has to let the users know somehow, encouraging the users to input a longer text. Probably for now a snackbar will do.