Calls to Spotify API can raise exception when the response is not JSON

Report by alec in discord. When the status of request to fetch token from spotify isn't 200, The content-type is not json.

While we are still here, the `self.get()` suffers from the same problem, it tries to parse json despite the response not necessarily being one (mostly when response status is 5xx). Fixing the get req needs lots of other changes and error handling pretty much all over the api interface, because it is used in many places unlike post (which just occurs once).

_Originally posted by @npc203 in https://github.com/Cog-Creators/Red-DiscordBot/issues/5359#issuecomment-932720142_