fix(build_ebs_on_ec2.sh): Add --fail to all curl commands.

This is particularly important for the image availability pre-check
because without it we don't detect that the image is in-fact unavailable
when it doesn't exist and the 404 results in a error from bzip2.
Merge pull request #83 from marineam/curl-ami-fix

fix(build_ebs_on_ec2.sh): Add --fail to all curl commands.