v1.21.1 — a transient failure can't wedge the card

The SmarterZones card had a failure where one bad render left the card broken until a browser reload. This card grew the same signature-cached rendering and had the same hole — I reproduced it: injecting a single error mid-build left the grid's cached signature set but its DOM empty, so every later good update matched the signature, patched an empty container, and never rebuilt. The card stayed blank for good.

Three fixes:

• set hass now runs in a try/catch. On any error it shows a small error card and clears its built flag, so the next state update rebuilds from scratch — instead of the raw exception reaching Home Assistant and breaking the card.
• Every cached container records its signature after the build succeeds, not before, so a mid-build failure leaves the old signature and the next pass rebuilds.
• Discovery no longer trips over a state whose attributes is null — the kind of half-formed state you can briefly get right after a restart.

Verified in a browser: the case that used to stay blank forever now shows an error card during the failure and fully recovers on the next clean update.