HDPublicKey: need better error handling when deriving child key

https://github.com/handshake-org/hsd/blob/107ed2be9280bb4292d21330e7d1ace62da9a002/lib/hd/public.js#L162-L167

This code is here, I believe to satisfy this requirement of the [BIP32 spec](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#public-parent-key--public-child-key)

> In case parse256(IL) ≥ n or Ki is the point at infinity, the resulting key is invalid, and one should proceed with the next value for i.

However I just encountered an issue trying to derive RECEIVE `branch` (0) from an account, and the error that was swallowed there was actually: `Error: Entropy source not available.` which resulted in the code deriving the CHANGE `branch` (1) which for whatever reason did not encounter the same error.

I'm still trying to figure out what happened to the entropy source and why the second iteration succeeded at all - but for hsd, I think we need to be more careful about checking the error thrown by the crypto library before just incrementing the index and trying again.