Add a message to the map view if no map tiles are available

 If either no network, or no SD card, notify the user no map tiles are shown, but stumbling is ok.

This is the no network message (there is another message for no sd card storage available):
![screenshot_2014-09-29-13-36-27](https://cloud.githubusercontent.com/assets/5495083/4446067/53b0f9f2-47ff-11e4-9ff5-89ac9d988b92.png)