Display Error when uploading an XML configuration file larger than the max attachment size

When you try to upload an attachment which is larger than the configured maximum attachment size inside of JIRA then the Import Project Configuration page simply refreshes and does not state why the XML file was not uploaded.

With this change, Project Configurator will show an error message when the plugin does not receive the configuration file, which is usually caused by the attachment size limit, and suggests increasing it.

Atlasian Page on how to increase default attachment size: https://confluence.atlassian.com/adminjiraserver073/configuring-file-attachments-861253987.html