[custom-images] The custom image script does not check for internet connectivity

[cjac]

Some users report that the custom image script does not work and end up guessing the root cause. The first command which attempts to access network resources is gsutil, and so customers who have VPCs without internet access incorrectly believe and report that the issue stems from gsutil.

In order to avoid this misclassification of issues, we can assert internet connectivity early in the script and report a clear failure message.