raise BuildFormationError on build or delete

[bacongobbler]

fixes #490

I'm not too sure if this is the right approach, but I'm throwing out this PR to see what you guys think.

[bacongobbler]

Here's what the new output looks like:

deis-controller.local     - execute the ruby block wait-for-builder
deis-controller.local
deis-controller.local
deis-controller.local Chef Client finished, 72 resources updated
+ set +x
Running post chef run setup...
[...]
Please ensure that "deis-controller" is added to the Chef "admins" group.
><> deis register http://deis-controller.local:8000
username: bacongobbler
password:
password (confirm):
email: [email protected]
Registered bacongobbler
Logged in as bacongobbler
><> deis formations:create blarg --flavor=rackspace-ord
Creating formation... done, created blarg
Creating runtime layer... 400 BAD REQUEST
Detail:
No credentials provided

[coveralls]

Coverage remained the same when pulling 6ba78b2 on 490-formation-without-credentials into 6ab9df7 on master.