CloudWatch: Handle permissions error and update docs (#88524)

pkg/tsdb/cloudwatch/testdata/permissions-error-output.json

@@ -0,0 +1,16 @@
+[
+  {
+    "Messages": null,
+    "MetricDataResults": [
+      {
+        "Id": "a",
+        "Messages": [{
+          "Code": "Forbidden",
+          "Value": "Access denied when getting data - please check that you have the pi:GetResourceMetrics permission"
+        }],
+        "StatusCode": "Forbidden"
+      }
+    ],
+    "NextToken": null
+  }
+]