Use preserve to auto-recover from errors

[bquistorff]

Currently, in places like parallel_do, parallel_bs, parallel_sim, when there are errors we try to reload the dataset. This is both cumbersome (we need capture error, load dta, then delete temporary variables) and can cause small errors (I remember that depending on the error, we'd drop the sortedby). I think we should make use of the preserve as this will do the recovery automatically. I think once we've put together the results from the children we can cancel the preserved data with restore, not

[gvegayon]

Could you point out the parts of the code in which such happens specifically?

[bquistorff]

For example in parallel.ado, on error, we reload - clean-up, reload - clean-up, and in parallel_spliter similar clean-up,

[bquistorff]

Done in fd81aea.