Fix for unclear error message for invalid YAML merge include (#196).

tests/integration/config/test_load.py

@@ -1,5 +1,7 @@
 import sys
 
+import pytest
+import ruamel.yaml
 from flexmock import flexmock
 
 from borgmatic.config import load as module
@@ -38,3 +40,23 @@ def test_load_configuration_merges_include():
     )
 
     assert module.load_configuration('config.yaml') == {'foo': 'override', 'baz': 'quux'}
+
+
+def test_load_configuration_does_not_merge_include_list():
+    builtins = flexmock(sys.modules['builtins'])
+    builtins.should_receive('open').with_args('include.yaml').and_return(
+        '''
+          - one
+          - two
+        '''
+    )
+    builtins.should_receive('open').with_args('config.yaml').and_return(
+        '''
+        foo: bar
+        repositories:
+          <<: !include include.yaml
+        '''
+    )
+
+    with pytest.raises(ruamel.yaml.error.YAMLError):
+        assert module.load_configuration('config.yaml')