Failing tests are not related to this pull request
This feature's small enough that I will probably merge it during the next pass of open tickets :) Thanks!
Re: @humanfromearth 's comment on #1089, one response to this could also be "this should always result in an error". However, I am usually a fan of "let you shoot yourself in the foot" features provided they are 100% opt-in and don't cause too much branching in existing code, and this does fit that bill. So it's still on the table for now.
Add option to skip unknown tasks
Set env.skip_unknown_tasks default
I've rebased master and fixed conflicting changelog to #1090.