Confusing error when nuisance regressors indents are misaligned in pipeline config

[sgiavasis]

When the nuisance regressor nested config items in the pipeline config are not properly indented, you get this non-informative error:

Traceback (most recent call last):
  File "/code/run.py", line 360, in <module>
    c = load_yaml_config(args.pipeline_file, args.aws_input_creds)
  File "/code/run.py", line 62, in load_yaml_config
    config_data = yaml.safe_load(open(config_filename, 'r'))
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/__init__.py", line 162, in safe_load
    return load(stream, SafeLoader)
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/__init__.py", line 114, in load
    return loader.get_single_data()
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/constructor.py", line 41, in get_single_data
    node = self.get_single_node()
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/composer.py", line 36, in get_single_node
    document = self.compose_document()
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/composer.py", line 55, in compose_document
    node = self.compose_node(None, None)
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/composer.py", line 84, in compose_node
    node = self.compose_mapping_node(anchor)
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/composer.py", line 133, in compose_mapping_node
    item_value = self.compose_node(node, item_key)
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/composer.py", line 82, in compose_node
    node = self.compose_sequence_node(anchor)
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/composer.py", line 111, in compose_sequence_node
    node.value.append(self.compose_node(node, index))
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/composer.py", line 84, in compose_node
    node = self.compose_mapping_node(anchor)
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/composer.py", line 127, in compose_mapping_node
    while not self.check_event(MappingEndEvent):
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/parser.py", line 98, in check_event
    self.current_event = self.state()
  File "/usr/local/miniconda/lib/python3.7/site-packages/yaml/parser.py", line 439, in parse_block_mapping_key
    "expected <block end>, but found %r" % token.id, token.start_mark)
yaml.parser.ParserError: while parsing a block mapping
  in "/configs/pipe_config.yml", line 845, column 4
expected <block end>, but found '<block mapping start>'
  in "/configs/pipe_config.yml", line 858, column 5