Monitoring: Backup: Clearer coloring

Failed backups now have a filled red area instead of a thin
yellow line.

Refs privatestorageops#429.

nixos/modules/monitoring/server/grafana-dashboards/backups.json

@@ -84,7 +84,7 @@
             "axisPlacement": "auto",
             "barAlignment": 0,
             "drawStyle": "line",
-            "fillOpacity": 0,
+            "fillOpacity": 60,
             "gradientMode": "none",
             "hideFrom": {
               "legend": false,
@@ -114,12 +114,31 @@
               {
                 "color": "green",
                 "value": null
+              },
+              {
+                "color": "red",
+                "value": 80
               }
             ]
-          },
-          "unit": "short"
+          }
         },
-        "overrides": []
+        "overrides": [
+          {
+            "matcher": {
+              "id": "byName",
+              "options": "Failed"
+            },
+            "properties": [
+              {
+                "id": "color",
+                "value": {
+                  "fixedColor": "semi-dark-red",
+                  "mode": "fixed"
+                }
+              }
+            ]
+          }
+        ]
       },
       "gridPos": {
         "h": 7,
@@ -224,7 +243,7 @@
             "axisPlacement": "auto",
             "barAlignment": 0,
             "drawStyle": "line",
-            "fillOpacity": 0,
+            "fillOpacity": 60,
             "gradientMode": "none",
             "hideFrom": {
               "legend": false,
@@ -259,7 +278,23 @@
           },
           "unit": "short"
         },
-        "overrides": []
+        "overrides": [
+          {
+            "matcher": {
+              "id": "byName",
+              "options": "Failed"
+            },
+            "properties": [
+              {
+                "id": "color",
+                "value": {
+                  "fixedColor": "semi-dark-red",
+                  "mode": "fixed"
+                }
+              }
+            ]
+          }
+        ]
       },
       "gridPos": {
         "h": 7,