Log error if invalid Bitmap passed to `FlxG.addBitmap()`

Resolves the following issue:
 * FlxG.as line 738,743 undefined property bitmapData
   - https://github.com/FlixelCommunity/flixel/issues/15
   - https://github.com/AdamAtomic/flixel/issues/217
Merge pull request #127 from IQAndreas/dev-flxg-addbitmap-warning

Log error if invalid Bitmap passed to `FlxG.addBitmap()`