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

internal/handlers/templates/dashboard.html

@@ -146,7 +146,11 @@
                 <td
                   class="py-2 px-3 text-slate-400 break-all max-w-xs"
                 >
+                  {{ if $nsr.Error }}
+                  <span class="text-red-400">{{ $nsr.Error }}</span>
+                  {{ else }}
                   {{ formatRecords $nsr.Records }}
+                  {{ end }}
                 </td>
                 <td class="py-2 px-3 text-slate-500 whitespace-nowrap">
                   {{ relTime $nsr.LastChecked }}
@@ -258,6 +262,11 @@
                   >
                   {{ end }}
                 </td>
+                {{ if $cs.Error }}
+                <td colspan="3" class="py-2 px-3 text-red-400 break-all">
+                  <div class="max-w-xs">{{ $cs.Error }}</div>
+                </td>
+                {{ else }}
                 <td class="py-2 px-3 text-slate-200">
                   {{ $cs.CommonName }}
                 </td>
@@ -285,6 +294,7 @@
                     {{ end }}
                   {{ end }}
                 </td>
+                {{ end }}
                 <td class="py-2 px-3 text-slate-500 whitespace-nowrap">
                   {{ relTime $cs.LastChecked }}
                 </td>