Bug: Plugin configuration missing `priority` value will still render Plugin, but disrupts ordering

[jsnwesson]

In the example app, I noticed that if a PLUGIN_OPERATION.Insert was missing a priority value, then all of the plugins in the slot would still render, but they would not appear in the order defined in the env.config.js.

Example:

 pluginSlots: {
    slot_with_insert_operation: {
      keepDefault: true,
      plugins: [
        {
          op: PLUGIN_OPERATIONS.Insert,
          widget: {
            id: 'inserted_direct_plugin',
            type: DIRECT_PLUGIN,
            // PRIORITY VALUE IS REMOVED
            RenderWidget: PluginDirect,
          },
        },
        {
          op: PLUGIN_OPERATIONS.Insert,
          widget: {
            id: 'inserted_iframe_plugin',
            type: IFRAME_PLUGIN,
            priority: 30,
            url: 'http://localhost:8081/plugin_iframe',
            title: 'The iFrame plugin that is inserted in the slot',
          },
        },
      ],
    },

Suggested effect:
If the priority is missing from a plugin config, the plugin should either:

1. Not be rendered, but then any other plugin operations that manipulate that Insert-ed plugin should be ignored/removed. In development, the logged error in the browser console will at least indicate that there's a misconfiguration.
2. Should render, but be placed at the end of the plugin ordering. This will make it obvious during development that it isn't rendering as expected, but it doesn't make it clear why that is the case even with the logged error in the browser console.