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/status.go

@@ -18,6 +18,7 @@ type statusDomainInfo struct {
 type statusHostnameNSInfo struct {
 	Records     map[string][]string `json:"records"`
 	Status      string              `json:"status"`
+	Error       string              `json:"error,omitempty"`
 	LastChecked time.Time           `json:"lastChecked"`
 }
 
@@ -41,6 +42,7 @@ type statusCertificateInfo struct {
 	NotAfter                time.Time `json:"notAfter"`
 	SubjectAlternativeNames []string  `json:"subjectAlternativeNames"`
 	Status                  string    `json:"status"`
+	Error                   string    `json:"error,omitempty"`
 	LastChecked             time.Time `json:"lastChecked"`
 }
 
@@ -144,6 +146,7 @@ func buildHostnames(
 			info.Nameservers[ns] = &statusHostnameNSInfo{
 				Records:     recs,
 				Status:      nsState.Status,
+				Error:       nsState.Error,
 				LastChecked: nsState.LastChecked,
 			}
 		}
@@ -183,6 +186,7 @@ func buildCertificates(
 			NotAfter:                cs.NotAfter,
 			SubjectAlternativeNames: sans,
 			Status:                  cs.Status,
+			Error:                   cs.Error,
 			LastChecked:             cs.LastChecked,
 		}
 	}