do not print a proxy's error page at somebody

first real test of the settings form returned openresty's 502 Bad Gateway
document, markup and all, into the panel. installing this plugin restarts
officer, and a request made inside that window reaches nginx proxy manager,
finds nothing behind it, and gets a web page back. useClient throws the
response body as the message, so the panel printed it.

worth keeping the irony: this plugin manages the proxy whose error page it
could not read.

the check is the body's SHAPE, not the status, because there is no status that
separates them — api/router.ts uses 502 for "NPM answered and refused", which
is a real answer we must not swallow. ours arrives as a sentence, theirs as a
document. gateway status AND html means the hop in front of officer answered;
html on its own still is not a sentence and says so without the tags.

all three panels go through it now.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Officer-Session: 656f5c85-e49a-4f1f-b9f3-042a04b4d043

web/HostsPanel.tsx

@@ -3,6 +3,7 @@ import { AlertTriangle, Globe, Lock, LockOpen, Power, RefreshCw, Settings, Shiel
 import { useClient } from "hooks/useClient";
 import { useSearchParams } from "react-router";
 import { ConnectionPanel } from "./ConnectionPanel";
+import { readableError } from "./errors";
 
 /**
  * The proxy hosts, which is the thing anybody opens this plugin to see.
@@ -42,7 +43,7 @@ type Failure = { error: string; likelyAuth?: boolean; unreachable?: boolean; unc
  */
 const asFailure = (error: unknown): Failure => {
   const status = (error as { status?: number })?.status;
-  const message = error instanceof Error ? error.message : "Could not load hosts.";
+  const message = readableError(error, "Could not load hosts.");
   return {
     error: message,
     unconfigured: status === 409,