Errors: Handle Auth Failure Conditions

As a person using the @dwyl app to be more personally effective,
I do not want to stumble at the first hurdle trying to authenticate.
I don't _expect_ to see unfriendly/unrecoverable error messages,
rather I expect to be able to _recover_ from errors without drama.

At present our `get_token/1` function is only following the "happy path":
https://github.com/dwyl/elixir-auth-google/blob/687fba552db8b5de2d353d42f77e2f04d9d428f9/lib/elixir_auth_google.ex#L18-L29

The callback function is `parse_body_response/1` which does not handle the `{:error, err}`

https://github.com/dwyl/elixir-auth-google/blob/687fba552db8b5de2d353d42f77e2f04d9d428f9/lib/elixir_auth_google.ex#L38-L45

This is _fine_ during MVP because as early ["dogfooding"](https://en.wikipedia.org/wiki/Eating_your_own_dog_food) users we are _tolerant_ of the failure/errors.
But as soon as we _ship_ and show the MVP to (_friendly_) alpha test users, we need to have this done.

# Todo:

+ [ ] Research and document the various failure conditions and HTTP status codes
+ [ ] Add failure `case` statement to handle all the error conditions.
+ [ ] Update the `template` to ensure that error conditions are displayed in a friendly way.
+ [ ] Update instructions in `README.md` > How? section to inform people about error scenarios.
+ [ ] Re-publish the hex.pm package with the error handling.

We do not need to handle the failures _before_ we ship our MVP, let's come back to this when it's needed.