If you specify a CUSTOM_PROJECTS_LIST_URL that doesn't exist you get a
message but the script continues. Since you clearly meant to specify
something we should bail so the user can correct the problem.
Additionally, add some documentation to the build-image.sh script to
describe how to use this to cut down build times considerably for
initial testing purposes.
Change-Id: I02d1cd341f793471b1ad4374e98031ea53db7f49