Add meaningful error codes and constants.

Replace literals with named constants in several pieces of code
like 'return -1' and 'case 0xab'.
Change the way decoder handles absence of image data in a file:
notify gif_decode_frame() caller with got_picture set to zero
instead of returning -1.

Signed-off-by: Vitaliy E Sugrobov <vsugrob@hotmail.com>