Throw on NaN without throwing on more than 15 significant digits

[subos2008]

I'm using DEBUG=true to convert NaN values into exceptions. However, there are other side effects of DEBUG=true that I pick up from using that.

Is it possible to convert NaN's to exceptions without other side effects?

[MikeMcl]

Well, you could just use, for example:

x = new BigNumber(val);
if (x.isNaN()) {
  throw Error(`${val} is not a valid BigNumber value`);
}

Related to #246.

I'm considering removing the 15 significant digit limit and throwing on NaN in DEBUG mode in the next release.