Improve error messaging around missing access token

[Cliabhach]

This specifically concerns the android part of this library. May belong under #40 , but I thought I'd open a new issue just in case.

I accidentally placed an access token in the wrong part of my android project's AndroidManifest. It was only when I noticed the NullPointerException error from https://github.com/tobrun/flutter-mapbox-gl/blob/26e3fec19e165cd3dcd6383555e5e99cbe2c08f5/android/src/main/java/com/mapbox/mapboxgl/MapboxMapController.java#L135-L139 in the logs and cross-referenced that with the README.md text that I discovered my mistake.

I suggest that those log messages should instead say something like

Failed to find an Access Token in the Application meta-data. MapBox may not load correctly. Please refer to the installation guide at (url goes here) for troubleshooting advice.