*FromBytes functions cannot return errors from NewMutable/NewImmutable

[l0k18]

I wanted there to be an error, because I wrote a handler clause, but then I looked at it and because these functions derive their size parameter from a len() function, they implicitly can never return a negative value, and newContainer only has this one error.

[awnumar]

The length can be zero in which case an error will be returned.