Calendars / Show error message and backtrace when synchronization fails

[ferblape]

When calendar synchronization fails the UI is not showing any error message, an admin can only notice when was happened the last successful sync.

It could be very useful to:

• show the last sync timestamp and the status (failed or success)
• if it failed, include a backtrace of the error for debugging purposes. The backtrace could consist in the error message and a collapsed text with the rest of the backtrace.