MM-51786: Sentry crash while installing a blocked plugin (#22804)

We would return "nil, nil" if a plugin was on the blocklist.
This would cause a nil dereference panic while trying to add
struct fields from manifest to the audit logs.

To avoid it, we return an error explicitly and ignore the
error ids to unnecessarily have it log an error.

https://mattermost.atlassian.net/browse/MM-51786

```release-note
NONE
```

server/i18n/en.json

@@ -5931,6 +5931,10 @@
     "id": "app.oauth.update_app.updating.app_error",
     "translation": "We encountered an error updating the app."
   },
+  {
+    "id": "app.plugin.blocked.app_error",
+    "translation": "Plugin {{.Id}} is on the block list. Some plugins are blocked because they are built into this version of Mattermost."
+  },
   {
     "id": "app.plugin.cluster.save_config.app_error",
     "translation": "The plugin configuration in your config.json file must be updated manually when using ReadOnlyConfig with clustering enabled."
@@ -6063,6 +6067,10 @@
     "id": "app.plugin.signature_decode.app_error",
     "translation": "Unable to decode base64 signature."
   },
+  {
+    "id": "app.plugin.skip_installation.app_error",
+    "translation": "Skipping installation of plugin {{.Id}} since existing version is equal or newer."
+  },
   {
     "id": "app.plugin.store_bundle.app_error",
     "translation": "Unable to store the plugin to the configured file store."