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 {

public/js/plugins.js

@@ -83,12 +83,23 @@ export class PluginsControl {
         east: bounds.getEast(),
       });
       const response = await fetch(`/api/plugins/${encodeURIComponent(plugin.name)}/run`, { method: "POST", body });
-      if (!response.ok) throw new Error(`run failed: ${response.status}`);
+      // Every app.rb response on this route is JSON, success or error —
-      const { path } = await response.json();
+      // the catch below is only for an infra-level failure in front of it
-      notifyGpxStoreChanged(path);
+      // (a proxy's own HTML error page, a dropped connection) that never
+      // reached app.rb at all.
+      const data = await response.json().catch(() => ({}));
+      if (!response.ok) throw new Error(data.error || `run failed: ${response.status}`);
+      notifyGpxStoreChanged(data.path);
       status.textContent = "Done";
+      status.classList.remove("plugins-panel__status--error");
     } catch (err) {
-      status.textContent = "Failed — see server log";
+      // Plugins::RunError's message (e.g. "plugin exited with status 1")
+      // reaches here as `data.error` above — nothing about a plugin run
+      // failure gets logged server-side (app.rb turns it into a plain 502
+      // JSON response, not an exception), so showing that message directly
+      // is the only way to see why, short of SSHing in to reproduce it.
+      status.textContent = err.message;
+      status.classList.add("plugins-panel__status--error");
     } finally {
       button.disabled = false;
     }