Draw status badges from a parked block's held outcome - #491

Summary

• Since Report a parked block's last check and return its result blockr.core#364, core reports a parked block by the outcome of its last check, stale once that is out of date, or unevaluated without one, and never dormant. The dormant case in block_status_badge(), which returned NA so that a persistent renderer such as the blockr.dag node kept the badge it drew last, could no longer be reached. It goes, along with NA in the documented return value.
• Nothing changes at runtime against current core, and the updated tests pass on the old code too. A new test pins the outcome the issue describes through core's board server: a block checked off screen by an evaluate request reads its held failed once parked and draws the failed badge.
• An unevaluated block that has raised an error still draws the failed badge, as ready does. Its errors come from the expression server, which runs while the block is parked, so unlike a stale block's they cannot predate a check.
• The comment on the stale exemption no longer says the change that made a block stale came from upstream, since an own edit now counts too. The unreleased Render the new stale eval status on the block status badge #408 NEWS entry says "parked" where it said "dormant", as core did for its own unreleased entries.

Fixes #485