Little cleanups.
More clean up of comments and imports.
Allow zooming programmatically.
Check if container is defined when setting marker pos.
It's ok if the tile server is not available.

If the tile server is not available because we are offline
or the server is down, don't raise exceptions: the standard
logic will provide a default blank tile.
Improve the aspect of the default blank tile.

It now contains a useful message.
Merge branch 'handle-offline'