jqueue: fix a bug in an error path
jqueue: fix a bug in an error path

Dictionaries raise KeyError, and not ValueError when invalid keys are
passes to del.

Reviewed-by: imsnah

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