Serverlist enhancements

[ShadowNinja]

This makes two enhancements for the server list:

• Print HTTP response bodies on error responses. This is useful because the server list returns information on why the request failed in the body. Without this you only get the response code, which could mean several different things.
• Add a world UUID and send it to the server list. This allows the server list to keep better track of worlds and is a companion to Persist servers in separate database serverlist#45 . This should not be merged until that PR is in production so that the server list doesn't publish UUIDs, which could be used to spoof servers.

How to test

Change src/serverlist.cpp so that it sends invalid JSON, enable announce, and start a server.

[rubenwardy]

Closing as no response in a long time. Feel free to reopen

[ShadowNinja]

One change to HTTP Fetch that would be visible to the Lua API is that HTTP responses with a 400+ code doesn't cause succeeded to be set to false anymore, only an actual cURL failure (e.g. connection refused) will cause that. lua_api.txt doesn't define what precisely succeeded means, though, so either implementation could be correct. Let me know what you think should be done.

[rubenwardy]

What remains to be done?

[sfan5]

Probably ready, the serverlist PR needs to be merged first.