Improve error message for `subseq` when called on a vector

[ahl27]

We could improve the error handling of subseq when called with multiple input args:

> seq <- DNAStringSet("ACGT")
> subseq(seq, 1:2, 2:3) # error
Error in solveUserSEW(x_eltNROWS, start = start, end = end, width = width) : 
  'start', 'end' or 'width' is longer than 'refwidths'
> subseq(rep(seq, 2), 1:2, 2:3) # no error
DNAStringSet object of length 2:
    width seq
[1]     2 AC
[2]     2 CG

Ideally the error message would be more informative, and we wouldn't be exposing solveUserSEW error calls.