Squashed commit of the following:

commit 0014ab17f2e0986044327a5ab22159de50b81e0a
Author: Andreas Huber <andih@google.com>
Date:   Thu Jan 14 14:05:09 2010 -0800

    Another instance of returning an error instead of asserting makes sense.

commit d2cac8c498a65b449a25ec216601830d23c165e7
Author: Andreas Huber <andih@google.com>
Date:   Thu Jan 14 13:34:33 2010 -0800

    Instead of asserting on invalid data (incomplete NAL unit), return an error.