Augment `TyCtxt::err` to be available via `TyCxAt`

[mark-i-m]

#70551 (comment)

Also it might be cool to use the TyCxAt helper to have tcx.at(span).err() use the span for the delay_span_bug, since I assume many of the callers of tcx.err() have a span around.

cc @eddyb