Capture and expose notification delivery errors

[santihernandezc]

This PR makes it possible to store the last error for each receiver in case of notification delivery failure. These errors are exposed via the /api/v2/receivers endpoint.

[diorcety]

There is an issue with this merge:

+                       if _, found := activeReceiversMap[rcv.Name]; !found {
                                // No need to build a receiver if no route is using it.
                                level.Info(configLogger).Log("msg", "skipping creation of receiver not referenced by any route", "receiver", rcv.Name)
+                               receivers = append(receivers, notify.NewReceiver(rcv.Name, false, nil))
                                continue
                        }

       "required": [
-        "name"
+        "name",
+        "active",
+        "integrations"
       ],

Either the required is incorrect (can't tell) or at couple of places there is missing code. For example if active/integrations are required, AlertToOpenAPIAlert should be fixed !

@santihernandezc what do you think?