More incremental output from tasks

[robin-aws]

V1 does the bare minimum of eventually showing you things like verification errors if a task fails, but it would be great to get more incremental output if tasks take multiple minutes.

May require some Dafny-side enhancements to do well: it's common to do --boogie /trace but that causes compatibility issues between .doo files since we require an exact match on --boogie (since it could be hiding verification-relevant knobs)