Fix ganeti-rapi startup with missing certificate
Fix ganeti-rapi startup with missing certificate

This patch displays a nicer error message compared to the default
stacktrace.

Reviewed-by: imsnah

mbakke/ganeti,mbakke/ganeti,ganeti/ganeti,ganeti/ganeti,leshchevds/ganeti,onponomarev/ganeti,bitemyapp/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,dimara/ganeti,ganeti/ganeti,yiannist/ganeti,grnet/snf-ganeti,mbakke/ganeti,yiannist/ganeti,andir/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,apyrgio/snf-ganeti,dimara/ganeti,onponomarev/ganeti,leshchevds/ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,andir/ganeti,andir/ganeti,leshchevds/ganeti,apyrgio/snf-ganeti,apyrgio/ganeti