Redesign Exception

```haskell
data OidcFlowException =
      DiscoveryFailed Text
    | InternalHttpError HttpException
    | JwtError JwtError
    | ValidationFailed Text    -- TODO: details
    | InvalidJsonData Text
  deriving (Show, Typeable)

foo :: (MonadThrow m, MonadIO m, Throws OidcFlowException) => m ()
foo = ...
```