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/channels/app/plugin_install.go

@@ -92,7 +92,10 @@ func (ch *Channels) installPluginFromData(data model.PluginEventData) {
 
 	manifest, appErr := ch.installPluginLocally(reader, signature, installPluginLocallyAlways)
 	if appErr != nil {
-		mlog.Error("Failed to sync plugin from file store", mlog.String("bundle", plugin.path), mlog.Err(appErr))
+		// A log line already appears if the plugin is on the blocklist or skipped
+		if appErr.Id != "app.plugin.blocked.app_error" && appErr.Id != "app.plugin.skip_installation.app_error" {
+			mlog.Error("Failed to sync plugin from file store", mlog.String("bundle", plugin.path), mlog.Err(appErr))
+		}
 		return
 	}
 
@@ -330,8 +333,8 @@ func (ch *Channels) installExtractedPlugin(manifest *model.Manifest, fromPluginD
 
 	// Check plugin id is not blocked
 	if plugin.PluginIDIsBlocked(manifest.Id) {
-		mlog.Debug("Skipping installation of plugin since plugin is on blocklist", mlog.String("plugin_id", manifest.Id))
-		return nil, nil
+		mlog.Debug("Skipping installation of plugin since plugin is on blocklist. Some plugins are blocked because they are built into this version of Mattermost.", mlog.String("plugin_id", manifest.Id))
+		return nil, model.NewAppError("installExtractedPlugin", "app.plugin.blocked.app_error", map[string]any{"Id": manifest.Id}, "", http.StatusInternalServerError)
 	}
 
 	// Check for plugins installed with the same ID.
@@ -365,7 +368,7 @@ func (ch *Channels) installExtractedPlugin(manifest *model.Manifest, fromPluginD
 
 			if version.LTE(existingVersion) {
 				mlog.Debug("Skipping local installation of plugin since existing version is newer", mlog.String("plugin_id", manifest.Id))
-				return nil, nil
+				return nil, model.NewAppError("installExtractedPlugin", "app.plugin.skip_installation.app_error", map[string]any{"Id": manifest.Id}, "", http.StatusInternalServerError)
 			}
 		}