Replace errNegativeNotAllowed with overflow error

[Márk Sági-Kazár]

Technically, the following is valid:

var i int64 = -1
j := uint64(i)

The value will overflow, but the conversion happens.

We should replace the errNegativeNotAllowed error with one that signals that the conversion was successful, but an overflow occurred.

Combining this idea with #86, it might even make sense to have a single base error.