Validate fromV3 params

[alcuadrado]

@michaelsbradleyjr commented in this PR that more validations in fromV3 would be useful:

I do think it would be a good idea to do a bit more options checking for uuid, iv, and salt options, i.e. to make sure that they're the expected length and valid hex (per the discussion in Discord last Thursday). Can we introduce that improvement in another PR?

I'm opening this issue to track the suggestion and any possible implementation.

[michaelsbradleyjr]

I believe the more pressing need is for validations in toV3, and that's now covered pretty well in #95. The reason for the need is that the underlying kdf functions are flexible enough to take bad inputs and generate legit-looking wallets that can't be unencrypted with fromV3.

We could also validate properties within the JSON/object passed to fromV3 and provide helpful error messages to signal to the user that the wallet is malformed (e.g. has a iv property that's too long/short or not valid hex) but I don't think the benefit is substantial. Let me know, though, if you'd like to see a PR for that.