Show the actual plugin run error instead of a generic message

A failed run showed "Failed — see server log" regardless of why —
misleading, since a Plugins::RunError never actually reaches any log
(app.rb turns it straight into a 502 JSON response). Found this the
hard way debugging the p4n2gpx plugin, having to SSH in and reproduce
the run by hand each time just to see why it failed. Now shows the
response's actual error message (e.g. "plugin exited with status 1"),
wrapping instead of overflowing the flyout for a longer one.

public/css/style.css

@@ -1487,8 +1487,17 @@ html, body {
 
 .plugins-panel__status {
   min-width: 3.5em;
+  max-width: 200px;
   color: #888;
   font-size: 11px;
+  /* Overrides the parent's nowrap: "Running…"/"Done" never wrap either
+     way, but a real RunError message (e.g. "plugin exited with status 1")
+     needs to, or it just overflows the flyout unreadably. */
+  white-space: normal;
+}
+
+.plugins-panel__status--error {
+  color: #c62828;
 }
 
 .plugins-panel__empty {