Notary should fail when being run in a script and no credentials provided

[dave-tucker]

I'd like to use notary in a Makefile to check that an specific org/image:tag is signed.
There are cases where the image may not have yet been pushed and in these cases I would expect notary to exit 1 so I can push/sign the image in question.

For example, in LinuxKit, if a contributor where to add a new package called "foo", our Makefile would check:

notary -s https://notary.docker.io lookup docker.io/linuxkit/foo $(HASH)

but linuxkit/foo would not yet exist.

This appeared to hang forever, but when I ran the command manually I saw that notary was prompting for a Username: and Password:. My expection would be that if no credentials were provided then this command should fail.

/cc @riyazdf

[riyazdf]

closed by #1200