GUI: Improve automatic determination of TW_THEME

* If TW_THEME is not defined, then check both DEVICE_RESOLUTION and
  TARGET_SCREEN_WIDTH x TARGET_SCREEN_HEIGHT for width/height. Use
  the determined width and height to automatically select landscape,
  portrait, or watch, as well as associated mdpi/hdpi.

* Improve the display of the error message so it fits in a standard 80
  character width terminal without line wraps.

* Write relevant errors for missing themes that clarify what went
  wrong.

Change-Id: I94b02349ce38bf6a9cecde403d78ff2490821d8a