RFE: dump-logs: error

Currently it seems stack supports `dump-logs: warning`, but I think often one is more interested in just the errors than warnings.
So it would be really useful to also have `dump-logs: error` for this and it looks pretty straightforward to add.

One particular use-case is for Stackage curator: sometimes we see extremely long warnings output after a rebuild of everything: I don't think we really look at warnings but we are certainly interested in the errors and want to see those displayed.