The 15-second refresh caught every error with `load().catch(() => {})`,
so a control pane that had lost its server kept showing the last
snapshot as if it were current. A failed refresh now shows the error box
with "Live refresh failed. The data below is from <time>." and the
error, and the next successful refresh clears it, as load() already
clears the box.
The new test runs the served page script in a vm with a stand-in for the
browser APIs it uses, fails a refresh, and checks both transitions.
Fixes#3176
L4XB
0ae2fa06a4