dashboard and status API show why a check failed (closes #225)

/api/v1/status now gives `error` for each nameserver entry and
certificate entry whose status is `error`, copied from the state, which
already kept it. The dashboard shows that reason in place of the records
for a failed nameserver, which used to show the same `-` as one that
answered with no records, and across the CN, issuer and expiry cells
for a failed certificate, wrapped at a width of 20rem so the long TLS
error does not narrow the Endpoint column. The dashboard stylesheet is a
trimmed build, so the new markup uses only classes the page already had.
README Web Dashboard and HTTP API say so.

Model: opus-5-5

README.md

@@ -230,9 +230,11 @@ dnswatcher includes an unauthenticated, read-only web dashboard at the root URL
 
 - **Summary counts** for monitored domains, hostnames, ports, and certificates.
 - **Domains** with their discovered nameservers.
-- **Hostnames** with per-nameserver DNS records and status.
+- **Hostnames** with per-nameserver DNS records and status. For a nameserver
+  whose query failed, the reason is shown in place of the records.
 - **Ports** with open/closed state and associated hostnames.
-- **TLS certificates** with CN, issuer, expiry, and status.
+- **TLS certificates** with CN, issuer, expiry, and status. For a failed check,
+  the reason is shown in place of CN, issuer and expiry.
 - **Recent alerts** (last 100 notifications sent since the process started),
   displayed in reverse chronological order.
 
@@ -259,6 +261,10 @@ dnswatcher exposes a lightweight HTTP API for operational visibility:
 | `GET /api/v1/status`           | Current monitoring state      |
 | `GET /metrics`                 | Prometheus metrics, see below |
 
+In `/api/v1/status`, each nameserver entry and certificate entry whose `status`
+is `error` also has `error`, the reason, as in the state file (see State File
+Format).
+
 `/metrics` is served only when `DNSWATCHER_METRICS_USERNAME` is set, behind
 Basic Auth. It has the Prometheus Go client's default metrics only (Go runtime,
 process, and counts of `/metrics` requests); dnswatcher records no metrics of