catch broken server config and disable encryption app.
also write error message to the log
improved error message to refect openSSL PHP extension and stock openSSL configuration issues
more error messages which might be useful for the user to debug his server config
Merge pull request #4268 from owncloud/encryption_check_config

Encryption app, check config
Check expected type after JSON decode instead of checking what is not expected.

This will also allow "true" and "false" as filenames instead of only "null".