fix KeyError when billing permission requirements are unsatisfied

[sosimon]

Instead of throwing a KeyError like this:

* module.project-factory.module.project-factory.null_resource.preconditions: Error running command '/cft/workdir/test/fixtures/minimal/.terraform/modules/76c2ddba7cd279abe2254470a2c70bac/scripts/prec
onditions.sh \
           --credentials_path '' \
           --billing_account '<redacted>' \
           --org_id '<redacted>' \
           --folder_id '<redacted>' \
           --shared_vpc ''
       ': exit status 1. Output: Traceback (most recent call last):
         File "/cft/workdir/test/fixtures/minimal/.terraform/modules/76c2ddba7cd279abe2254470a2c70bac/scripts/preconditions/preconditions.py", line 455, in <module>
           retcode = main(sys.argv)
         File "/cft/workdir/test/fixtures/minimal/.terraform/modules/76c2ddba7cd279abe2254470a2c70bac/scripts/preconditions/preconditions.py", line 437, in main
           results.append(validator.validate(credentials))
         File "/cft/workdir/test/fixtures/minimal/.terraform/modules/76c2ddba7cd279abe2254470a2c70bac/scripts/preconditions/preconditions.py", line 298, in validate
           response["permissions"],
       KeyError: 'permissions'

The precondition script will output a proper error like this:

* module.project-factory.module.project-factory.null_resource.preconditions: Error running command '/cft/workdir/test/fixtures/minimal/.terraform/modules/76c2ddba7cd279abe2254470a2c70bac/scripts/prec
onditions.sh \
           --credentials_path '' \
           --billing_account '<redacted>' \
           --org_id '<redacted>' \
           --folder_id '<redacted>' \
           --shared_vpc ''
       ': exit status 1. Output: [
           {
        "type": "Required APIs on service account project",
        "name": "projects/cft-seed",
        "satisfied": [
            "appengine.googleapis.com",
            "iam.googleapis.com",
            "cloudresourcemanager.googleapis.com",
            "admin.googleapis.com",
            "cloudbilling.googleapis.com"
        ],
        "unsatisfied": []
           },
           {
        "type": "Service account permissions on billing account",
        "name": "billingAccounts/<redacted>",
        "satisfied": [],
        "unsatisfied": [
            "billing.resourceAssociations.create"
        ]
           },
           {
        "type": "Service account permissions on parent folder",
        "name": "folders/<redacted>",
        "satisfied": [],
        "unsatisfied": [
            "resourcemanager.projects.create"
        ]
           },
           {
        "type": "Service account permissions on organization",
        "name": "organizations/<redacted>",
        "satisfied": [],
        "unsatisfied": [
            "resourcemanager.organizations.get"
        ]
           }
       ]