Better error reporting when missing setup for E2E tests (#1985)

tests/fixtures.py

@@ -296,10 +296,18 @@ def plan_validator_fixture(request):
 
 def get_tfvars_for_e2e():
   _variables = [
-      "billing_account", "group_email", "organization_id", "parent", "prefix",
-      "region"
+      'billing_account', 'group_email', 'organization_id', 'parent', 'prefix',
+      'region'
   ]
-  tf_vars = {k: os.environ.get(f"TFTEST_E2E_{k}") for k in _variables}
+  missing_vars = set([f'TFTEST_E2E_{k}' for k in _variables]) - set(
+      os.environ.keys())
+  if missing_vars:
+    raise RuntimeError(
+        f'Missing environment variables: {missing_vars} required to run E2E tests. '
+        f'Consult CONTRIBUTING.md to understand how to set them up. '
+        f'If you want to skip E2E tests add -k "not examples_e2e" to your pytest call'
+    )
+  tf_vars = {k: os.environ.get(f'TFTEST_E2E_{k}') for k in _variables}
   return tf_vars