Validate inputs to `cog predict` before running model

[bfirsh]

And display help text. Might be a soft dependency on #205

[evilstreak]

I think this is resolved by #420

[bfirsh]

I think the intention of this is to test is before running the model, inside Go, instead of booting up the model and letting Pydantic deal with it.

[evilstreak]

Oh, that would be neat!