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.go

@@ -352,7 +352,7 @@ func (ch *Channels) syncPlugins() *model.AppError {
 			}
 
 			mlog.Info("Syncing plugin from file store", mlog.String("bundle", plugin.path))
-			if _, err := ch.installPluginLocally(reader, signature, installPluginLocallyAlways); err != nil {
+			if _, err := ch.installPluginLocally(reader, signature, installPluginLocallyAlways); err != nil && err.Id != "app.plugin.blocked.app_error" && err.Id != "app.plugin.skip_installation.app_error" {
 				mlog.Error("Failed to sync plugin from file store", mlog.String("bundle", plugin.path), mlog.Err(err))
 			}
 		}(plugin)
@@ -952,6 +952,11 @@ func (ch *Channels) processPrepackagedPlugins(pluginsDir string) []*plugin.Prepa
 			defer wg.Done()
 			p, err := ch.processPrepackagedPlugin(psPath)
 			if err != nil {
+				var appErr *model.AppError
+				// A log line already appears if the plugin is on the blocklist
+				if errors.As(err, &appErr) && (appErr.Id == "app.plugin.blocked.app_error" || appErr.Id == "app.plugin.skip_installation.app_error") {
+					return
+				}
 				mlog.Error("Failed to install prepackaged plugin", mlog.String("path", psPath.path), mlog.Err(err))
 				return
 			}