session/*: Fix Errors unhandled issues
session/*: Fix Errors unhandled issues

See,
$ gometalinter --vendor --disable-all --enable=gosec --deadline 10m ./session/...
session/keep_alive.go:43::warning: Errors unhandled.,LOW,HIGH (gosec)

bruceadowns/govmomi,vmware/govmomi,vmware/govmomi,bruceadowns/govmomi,vmware/govmomi,bruceadowns/govmomi