Clearer error message in case of no connectivity (#163)
This merge request addresses issue #163 (closed), it improves error messages when router has no connectivity and tries to run updater.
So far user got message "unreachable: /tmp/crl.pem: No such file or directory ". Missing CRL was not detected before trying to download files needed for update.
This adds check if the CRL file exists before proceeding further. If not, it doesn't proceed, it just notifies the user and dies.
The message for the user might be changed, this is just my initial proposal, I'm completely open to any change here.
This merge request depends on merge request turris/misc!16 (closed), which slightly changes exit codes of get-api-crl script.