Error handling for job cost script

- Exit early if on AWS, where job cost calculation isn't supported
- Error handling for vexhost API - may return a 404 which doesn't parse
  properly in jq, which would fails an otherwise working build.
- Report correct name when script is run.

Change-Id: I375b4a16588b1ab12e4aa7da0e37b351ed52f64c
Signed-off-by: Zack Williams <zdw@opennetworking.org>