bigNumberToByteArray: Avoid unexpected behaviour by throwing exception

[davidyuk]

Implementing #1060 I have found that passing a non-integer amount value looks working, but actually the amount value is gets broken instead of being rounded somehow. BigNumber.toString(16) returns a hex value separated with a dot for non-integer values that get passed to Buffer.from that just ignores the part after .:

> Buffer.from('390cf7311ac9f5ab9746ab.2ab9f559b3d07c84b5dd', 'hex')
<Buffer 39 0c f7 31 1a c9 f5 ab 97 46 ab>

Alignment by adding a leading 0 also may not work properly in this case.

bigNumberToByteArray is handling non-integers not properly, so I propose to throw an exception then.

[mradkov]

Sounds reasonable. Code change looks good. Is the exception being catched somewhere and what is the use-case where this is most likely the user to hit this case?

[davidyuk]

It looks like sdk uses bigNumberToByteArray properly in all cases, but the end-user may ask sdk to transfer for example 123.55 aettos and before this fix the sdk would send 7 aettos (BN(123.55) => 7b.8ccccccccccccccccccd => Buffer.from('07b.8cc...', 'hex') => [07]) and with this fix sdk will reject that attempt. I assume that the user will figure out that he needs to round the token value somehow after seeing the exception.