Revoking Token thats already been removed throws error rather than returning error

[rraub]

We've run into a condition where a request to refresh a token was replayed which resulted in duplicate requests processed at the same time. The first one succeeded and the second one failed with an Ecto.StaleEntryError when trying to revoke and delete the token. Instead of doing a delete on the Model could we just do a delete by the value of the primary key so we can return an {:error} instead of throwing an error?
Similar to:

guardian_db/lib/guardian/db/token.ex

Line 66 in 82fb31f

|> Guardian.DB.repo().delete_all(prefix: prefix())

[yordis]

@rraub would you mind to open a PR with the desired behavior?