Handle version check errors better.

[NotAFile]

Another such error

    File "/piqueserver/release.py", line 16, in fetch_latest_release
        return await response.json()
    File "aiohttp/client_reqrep.py", line 1026, in json
        raise ContentTypeError( aiohttp.client_exceptions.ContentTypeError: 0, 
        message='Attempt to decode JSON with unexpected mimetype: text/html', 
        url=URL('https://api.github.com/repos/piqueserver/piqueserver/releases/latest')

github:

Originally posted by @Jipok in #578 (comment)