call .raise_for_status() before .json() for certain requests

[talflon]

Addresses #15227. .json() on Metasmoke requests was causing an error like

requests.exceptions.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

so this calls raise_for_status() first, which will raise an HTTPError instead if it's a 4xx or 5xx HTTP error.

I also did this in chatcommands.pull(), because that method appears to assume a successful response.

I didn't do this for:

• the other GitHub API requests: seems we might rely on parsing the JSON body of an error response
• StackExchange API requests: seems we might rely on parsing the JSON body of an error response
• the ChatExchange call in chatcommunicate.sendmessages(): seems there is some error handling in ChatExchange
• findspam.toxic_check(): looks as if errors are returned as JSON
• Metasmoke.get() itself: MetasmokeCache.fetch_from_api() calls .get() and then handles the status code separately
• BodyFetcher.make_api_call_for_site(): all exceptions are swallowed, so makes no difference
• deletionwatcher._check_batch(): this code appears to have been obsolete for over a year