Use of `BCRYPTO_FORCE_BIGINT` may silently error and fallback to JS without BigInt

[braydonf]

It may be expected that using BCRYPTO_FORCE_BIGINT would always use BigInt or otherwise error. However this may not be the case, as it will fallback to use JS without BigInt, see: https://github.com/bcoin-org/bcrypto/blob/master/lib/bn.js