Provide a sensible diagnostic if a user forgets the tool

Previously, we defaulted to `atllbuild` in this case, and started
prompting them about missing atllbuild keys, e.g.

    fatal error: Unknown outputType nil

This is almost certainly the wrong thing to do, instead we should
provide a diagnostic requiring them to specify the tool.

Test coverage.

Close #41
3 jobs for agressive
Status Job ID Name Coverage
  Build
passed #1175
docker
linux

00:01:03

passed #1174
openswift
osx

00:00:16

failed #1173
docker
linux

00:00:00