Calendar:
- Undefined return values and empty strings from GetFileFromURLQuiet() are error conditions.
- Do not retry download of ical file immediately; wait until the next scheduled time instead.
- STATE is "No data" on error.

git-svn-id: svn://svn.code.sf.net/p/fhem/code@4695 2b470e98-0d58-463d-a4d8-8e2adae1ed80