Gracefully fail if application link is deleted

#Scenario 1

• Delete application link
• JIRA is still running

Result: 500 Error not encountered in Team Calendars in Confluence.

#Scenario 2

• Delete application link
• Emptying the cache

Result: 500 Error IS encountered in Team Calendars in Confluence.

#Scenario 3

• Re-create application link
• JIRA is shut down

Result: 500 Error not encountered in Team Calendars in Confluence

#Scenario 4

• Delete application link
• JIRA is shut down

Result: 500 Error IS encountered in Team Calendars in Confluence

#Scenario 5

• Startup JIRA
• Application link not created

Result: 500 Error IS encountered in Team Calendars in Confluence

#Scenario 6

• Startup JIRA
• Application link is re-created

Result: NO 500 Error in Team Calendars in Confluence

#Scenario 7

• JIRA is running
• Application link exist
• Delete a project in JIRA that is associated with a calendar in Confluence

Result: NO 500 Error in Team Calendars in Confluence

#Scenario 8

• JIRA is running
• Application link exist
• jira-ical-feed plugin is disabled

Result: Team Calendars complains of a connection problem - "You may want to check the connection to XXX".

Conclusion: 500 errors will occur if the application link is deleted.