Make client robust to server failure

[elliottslaughter]

Currently when the server fails for whatever reason (or a static profile is out of date), the client crashes. It would be better to make it robust and simply display an empty tile or an error.

It's worth thinking about a couple different failure modes (not that these are necessarily equally easy to address):

• User types the wrong URL or profile otherwise does not exist
• Add trailing slash to URL if not already present (Always add trailing slash to URL if not provided by user #83)
• A tile does not exist, the server or network fails, or the tile fails to decode (Handle errors when fetching tiles #80)
• Server crashes in the same process (probably much harder to deal with)