Underscores in app names not reported properly

If you have a snap with multiple apps, and one of the apps has an underscore in its name (p.s. it'd be nice to see something move on #1616507 :), then a confusing error is reported against the first app only, even if that is valid.

For example, with the attached snapcraft.yaml including an app "one" and "two_three", the error is:

Issues while validating snapcraft.yaml: The 'apps' property does not match the required schema: 'one' is not a valid app name. App names consist of upper- and lower-case alphanumeric characters and hyphens. They cannot start or end with a hyphen.