Added friendly handling/error message for nonexistent plugin

Fixes #1829

* Added try-catch block in `core/server/plugins/loader.js` to catch and reject with an error when `getPluginByName` fails (meaning `require(getPluginRelativePath(name))` failed, meaning the plugin directory doesn't exist).  Error message as follows:

> Error loading plugin named \<name>; plugin directory (/content/plugins/\<name>) does not exist."

* Added .otherwise blocks with error logs to `core/server/plugins/index.js` to catch the new rejections and log error message to console