Fix: Handle error in job-cost.sh without failing

We have seen builds get marked as "unstable" due to transient network
failures while retrieving instance type info. This is not an
acceptable criteria for marking a build unstable, so this change will
handle such an error more cleanly.

Issue: RELENG-4970
Change-Id: Ia146e2771df638e8410fb187d730d4faadf132c2
Signed-off-by: Eric Ball <eball@linuxfoundation.org>