Don't catch all exceptions in TileReader.get_embedded_image

The return value of TileReader.get_embedded_image is always passed into
base64.b64decode, which fails when passed None. This, an exception will
still always occur, but allowing it to propagate naturally from
get_embedded_image will provide better debugging information.
Merge branch 'ptiff_exception'

* ptiff_exception:
  Don't catch all exceptions in TileReader.get_embedded_image