Warn if imageWithoutTag wasn't found

[jonashaag]

Scenario: Image URLs are changed and ecs-deploy is fed a new image URL. Example: Old URL: foo-image:12345 New URL fed to ecs-deploy: foobar-image:23456

In this case ecs-deploy can't find the old image's tag to replace. But it fails silently without replacing any image URL/tag at all.

Root cause is that imageWithoutTag in the sed call doesn't find any matches.

ecs-deploy should abort with a nice error message.

[tonyswu]

Ran into this as well. If it's intended, then it should fail when SED does not have any matching string to replace when forming new task definition. But what would be the purpose though? I'd imagine it's within reason to want to replace an image repo during deployment.