alerting: fixes validation error when saving alerts in dash

If a panelId in the dashboard json is set to zero then the validation
silently fails. Instead of returning an error, it just ignores alerts and
saves the dashboard.

(cherry picked from commit d96fbb486f)

pkg/services/alerting/test-data/panel-with-id-0.json

@@ -0,0 +1,63 @@
+{
+  "id": 57,
+  "title": "Graphite 4",
+  "originalTitle": "Graphite 4",
+  "tags": ["graphite"],
+  "rows": [
+  {
+    "panels": [
+    {
+      "title": "Active desktop users",
+      "id": 0,
+      "editable": true,
+      "type": "graph",
+      "targets": [
+      {
+        "refId": "A",
+        "target": "aliasByNode(statsd.fakesite.counters.session_start.desktop.count, 4)"
+      }
+      ],
+      "datasource": null,
+      "alert": {
+        "name": "name1",
+        "message": "desc1",
+        "handler": 1,
+        "frequency": "60s",
+        "conditions": [
+        {
+          "type": "query",
+          "query": {"params": ["A", "5m", "now"]},
+          "reducer": {"type": "avg", "params": []},
+          "evaluator": {"type": ">", "params": [100]}
+        }
+        ]
+      }
+    },
+    {
+      "title": "Active mobile users",
+      "id": 4,
+      "targets": [
+        {"refId": "A", "target": ""},
+        {"refId": "B", "target": "aliasByNode(statsd.fakesite.counters.session_start.mobile.count, 4)"}
+      ],
+      "datasource": "graphite2",
+      "alert": {
+        "name": "name2",
+        "message": "desc2",
+        "handler": 0,
+        "frequency": "60s",
+        "severity": "warning",
+        "conditions": [
+        {
+          "type": "query",
+          "query":  {"params": ["B", "5m", "now"]},
+          "reducer": {"type": "avg", "params": []},
+          "evaluator": {"type": ">", "params": [100]}
+        }
+        ]
+      }
+    }
+    ]
+  }
+]
+      }