These can give misleading results. Checking the sum is cheap, but
checking for negative weights is relatively costly. Therefore, compute
this information the first time it is requested, and store it in
the weights vector like the sum.

`efraimidis_ares_wsample_norep!` and `efraimidis_aexpj_wsample_norep!` already
checked these, but throwing different exception types. Harmonize exceptions
across algorithms as they can all be called by `sample`.