Verify download of pre-trained model

[adamltyson]

If the download of the pre-trained model is interrupted, it can't be loaded by tensorflow and there's a fairly cryptic error message (e.g. OSError: Unable to open file (truncated file: eof = 62652416, sblock->base_addr = 0...) that doesn't help the user debug.

We could checksum, or even just catch the error. Catching the error could be used for any model/weights, not just the pre-trained model.