CLI: Handle error better
CLI: Handle error better

This patch adds an error handler for any exceptions that are raised
during the external data load phase. This can be improved further, but
it's a good start.

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