Pull missing artifacts in `bst artifact push`

As per #819, BuildStream should pull missing artifacts by default. This is consistent with bst source push fetching missing sources. The previous behavior was to only pull missing buildtrees. A top-level --no-pull option can easily be supported in the future.

This change makes it possible to use a single scheduler session (with concurrent pull and push jobs). This commit also simplifies the code as it removes the sched_error_action emulation, using the regular frontend code path instead.