[Audio] Spotify post request fix

[npc203]

Description of the changes

Report by alec in discord. When the status of request to fetch token from spotify isn't 200, The content-type is not json.
This PR handles it ,makes sure the error is propagated and handled.

[npc203]

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). However 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). Thus I didn't cramp it up into a single PR and let better people deal with it. but do let me know if you would like me to update this PR with a similar approach as done in post.

[Jackenmen]

We still should be checking for status code as some (if not most) non-200 responses contain valid JSON. Yes, with current usage this doesn't technically matter but we should be doing it nonetheless.
If we do this, we could probably even simplify get_access_token() and remove the try-except with KeyError from it and just directly assign instead.

[Jackenmen]

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). However 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). Thus I didn't cramp it up into a single PR and let better people deal with it. but do let me know if you would like me to update this PR with a similar approach as done in post.

I suppose this could be added to this PR as well, it's part of the same issue.